Primer3 0.4.0 (2026)

Primer3 0.4.0 uses a plain text keyword-value format, terminated by = on a line by itself.

SEQUENCE_ID=Test_Amplicon
SEQUENCE_TEMPLATE=ATGGCGTACTGACGGTACCTAGCTTAGGC
SEQUENCE_TARGET=10,20
PRIMER_TASK=generic
PRIMER_PICK_LEFT_PRIMER=1
PRIMER_PICK_RIGHT_PRIMER=1
PRIMER_PICK_INTERNAL_OLIGO=0
PRIMER_OPT_SIZE=20
PRIMER_MIN_SIZE=18
PRIMER_MAX_SIZE=27
PRIMER_OPT_TM=60.0
PRIMER_MIN_TM=57.0
PRIMER_MAX_TM=63.0
PRIMER_PRODUCT_SIZE_RANGE=100-300
=

import subprocess
import sys
def run_primer3(template, min_tm=57, max_tm=63):
inp = f"""SEQUENCE_ID=py_test
SEQUENCE_TEMPLATE=template
PRIMER_MIN_TM=min_tm
PRIMER_MAX_TM=max_tm
="""
proc = subprocess.run(['primer3_core'], input=inp, text=True, capture_output=True)
return proc.stdout

Primer3 0.4.0 computes seven core metrics for each candidate:

| Metric | Constraint Parameters | Default (v0.4.0) | |--------|----------------------|------------------| | Tm | PRIMER_MIN_TM, PRIMER_OPT_TM, PRIMER_MAX_TM | 57.0, 60.0, 63.0 | | GC% | PRIMER_MIN_GC, PRIMER_MAX_GC | 20.0, 80.0 | | Length | PRIMER_MIN_SIZE, PRIMER_OPT_SIZE, PRIMER_MAX_SIZE | 18, 20, 27 | | Self complementarity | PRIMER_MAX_SELF_ANY | 8.00 | | Self 3'-end complementarity | PRIMER_MAX_SELF_END | 3.00 | | Primer-dimer | PRIMER_MAX_PAIR_COMPL_ANY | 8.00 | | 3'-end stability | PRIMER_MAX_END_STABILITY | 9.0 | primer3 0.4.0

Behind the scenes, v0.4.0 migrated the codebase to C. While this sounds technical, it means the tool is faster, easier to maintain, and runs more reliably across different operating systems (Windows, macOS, Linux). This stability is why it is the engine of choice for high-throughput pipelines and web servers.

The tool rigorously enforces stability at the 3' end to prevent "primer-dimer" artifacts. v0.4.0 allows fine-tuning of PRIMER_MAX_END_STABILITY and PRIMER_MAX_POLY_X (limiting runs of a single nucleotide, e.g., GGGG). Primer3 0

One of the strengths of Primer3 v0.4.0 is its versatility.

For the Wet Lab Biologist: Tools like Primer3Web provide a user-friendly interface to the v0.4.0 engine. You simply paste your FASTA sequence, adjust the target region (highlighting the exon or SNP you want to amplify), and download the results. PRIMER_MAX_TM | 57.0

For the Bioinformatician: The command-line tool (primer3_core) is incredibly powerful for high-throughput analysis. You can feed it a file containing thousands of sequences (boulder-IO format) and automate the design of primers for an entire genome. This version is easily integrated into Python or Perl pipelines, making it essential for large-scale genotyping projects.