Assay-specific design workflows

Documentation checked: 2026-10-01. These are design and review procedures. The bundled tools automate ordinary DNA-primer design, thermodynamics, and bounded local-reference screening; they do not implement every specialized assay below.

Define success before choosing sequences

Write a design brief with:

• Assay purpose: genomic PCR, RT-PCR/RT-qPCR, cloning, genotyping, or another named assay; biological material and expected background DNA.
• Intended target set: one locus, one transcript, several named isoforms, or all members of an explicitly listed set. A gene symbol alone is insufficient.
• Reference identity: accession with version, assembly and annotation release, strand, extraction coordinates, FASTA checksum, and reference retrieval date.
• Desired product interval, region that must be contained in the product, regions primers must avoid, and any fixed primer or required junction.
• Polymerase/master mix, known reaction chemistry, available empirical validation, and constraints imposed by the downstream method.

Keep defaults provisional when chemistry is unknown. Do not translate a missing reference or ambiguous target into a made-up sequence. Acquire the sequence or provide the unfinished design specification and the missing inputs.

Ordinary genomic PCR

1. Extract the target with enough flanking sequence for alternative binding sites. Retain its mapping to the full reference; template-local coordinates are not chromosome coordinates.
2. Mark low-confidence sequence, repeats, and relevant population/sample variants as exclusions when their avoidance is part of the assay specification.
3. Design several candidates with explicit product and primer constraints. Inspect rejection reasons if none return; relax one justified constraint at a time and record each change.
4. Verify both synthesized primer sequences against the template, their inward orientation, and the entire expected product. Include primer-binding sites in the product length.
5. Screen against the broader reference and expected background. A successful design on a small extracted window establishes no genome-wide specificity.
6. Retain at least one alternative pair at different sites when feasible. Rank acceptable candidates by assay priorities, not solely by Primer3 penalty.

Keep intended product size distinct from the maximum unintended-product size screened. Search for plausible off-target products outside the intended design range as well. See specificity.md.

RT-PCR and RT-qPCR

Choose transcript-specific, gene-level, or explicit multi-transcript detection before designing. Enumerate intended and excluded transcript accessions; check that the selected annotation actually contains the junction of interest.

An exon-junction primer can reduce genomic amplification, but its binding site must be checked against genomic DNA and processed pseudogenes. A pair in different exons instead relies on intervening genomic structure; short introns can still produce genomic products. Test the annotated transcript sequence and corresponding genomic context separately. NCBI Primer-BLAST supports these distinctions when appropriate reference records are supplied. Its “allow splice variants” option permits additional variants; it does not require coverage of all variants. Primer-BLAST guidance.

For qPCR, select product dimensions appropriate to the chemistry, sample quality, and assay purpose; no size or Tm range guarantees an efficient assay. Plan experimental specificity checks, no-template controls, and a no-RT control where genomic carryover matters. Estimate efficiency and usable quantitative range with suitable material. A single melt peak is evidence to review with product identity, not proof that the desired transcript was amplified. Report sequences, product identity, chemistry, controls, and measured validation separately from predictions. MIQE 2.0.

Primer design does not establish stable reference-gene expression, valid normalization, or biological replication. Hydrolysis probes additionally require probe-specific chemistry and modification-aware analysis; a Primer3 internal oligo alone is not a validated probe assay.

Variants and allele discrimination

For a general assay, intersect both complete binding intervals with variants from the relevant assembly and population or sample. Inspect near-3′ variants closely, but retain the position and identity of every overlapping variant. Variant absence from a database is not evidence that a sample has no variation.

Use an explicit coordinate conversion before creating exclusions. For example, a 1-based SNP at reference position p occupies [p-1, p) in 0-based half-open coordinates. If a plus-strand extracted template starts at offset s, its local interval is [p-1-s, p-s). For reverse-complement extraction, additionally transform through the extracted sequence length. Confirm the reference allele after mapping. Indels require their complete affected interval and attention to normalization; do not treat every VCF record as a one-base exclusion.

For intended allele-specific PCR, state which allele each primer should recognize, retain both allele sequences, and predict binding/product formation on both. Deliberate 3′ discrimination needs validation with known positive and negative genotypes. Neither a terminal mismatch nor a numerical mismatch budget proves allele discrimination; this is a specialized assay, not an automatic “pass” from the ordinary specificity screen. The need to consider polymorphic binding sites is discussed in the Primer-BLAST paper.

Cloning primers and 5′ additions

Store three distinct sequences for each oligo: the annealing core, the 5′ tail, and the full ordered oligo (tail + core, all 5′→3′). The reverse primer is already supplied in synthesized orientation; do not reverse-complement its entire ordered sequence again.

Use the core for initial template binding and reference specificity. Assess the full oligo for hairpins and primer-primer interactions. Reconstruct the final double-stranded product explicitly: in forward orientation its ends are the forward tail and the reverse complement of the reverse tail. Verify every junction, orientation, coding frame, and required downstream sequence.

For restriction cloning, check recognition-site placement, internal sites, and the enzyme's current end-cleavage requirements. For overlap assembly, validate overlap identity and order against the neighboring fragment, not the source template. Assembly and PCR impose separate constraints; use the chosen reagent's current instructions. NEB assembly design guidance.

Very long tails may exceed the bundled thermodynamic engine's supported lengths. Report that gap; do not silently truncate the oligo. Circular plasmids also require an origin-aware product search rather than ordinary linear coordinates.

Multiplex panels

Validate each pair independently first. Then enumerate interactions across all oligos sharing a tube, including tails and cross-pair forward/reverse combinations. Assess cross-pair amplification on the reference in addition to dimers. A dimer matrix alone does not establish multiplex specificity.

Build a conflict table linking interacting oligo IDs, predicted structures, products, and proposed pool assignments. Reconsider conflicting pairs or split pools, then recompute the complete panel. Report omitted combinations and any analysis cap. Validate performance in the final mixture: favorable singleplex results do not establish multiplex efficiency or balanced yield. Pool assignment can be treated as a separate optimization problem, as in PrimerPooler.

Degenerate and family-wide primers

Start from a curated multi-sequence alignment and an explicit coverage target. Identify which variants each concrete oligo can bind. IUPAC codes describe mixtures; they are not ordinary bases for the bundled thermodynamic/design routines. For small, bounded mixtures, expand every concrete oligo and analyze all resulting species and products; record the expansion size and the assumed concentration per species. If expansion is capped, report incomplete coverage instead of selecting one arbitrary realization.

For protein-guided family discovery, CODEHOP uses a degenerate core and consensus clamp; it requires an appropriate alignment and specialized design procedure. Do not substitute ordinary single-template Primer3 design and claim equivalent family coverage. CODEHOP primary description.

Bisulfite and other specialized chemistries

Distinguish methylation-specific amplification from methylation-independent bisulfite sequencing. They have different requirements for CpGs in binding sites. Design on the appropriate converted sequence states and screen the corresponding converted reference strands; an untreated-reference search cannot establish specificity after conversion. Conversion reduces sequence complexity and can alter which strands are complementary. These workflows need dedicated conversion-aware tools and validation, such as those described in MethPrimer and BiSearch.

Modified oligos, LNA/PNA chemistries, nested PCR, long-range PCR, inverse PCR, and site-directed mutagenesis each add requirements beyond this skill's automatic ordinary-primer workflow. Retain the relevant chemistry or geometry explicitly, use its specialist method, and report what the bundled tools could not establish.

Candidate decision record

Deliver a ranked table with oligo IDs, 5′→3′ sequences, core/tail distinction, reference coordinates and strand, expected product, chemistry-specific Tm, structure flags, specificity evidence, and decision rationale. Include discarded candidates with concrete failure reasons when useful. Label the state precisely: designed, computationally screened within stated limits, or empirically validated under stated conditions. These are different evidence levels.