Three to One
Convert three-letter amino acid codes to single-letter codes
Input Sequence

Paste a raw three-letter sequence or one or more FASTA sequences. Digits and whitespace are stripped automatically. Stop codons: use ***. Input limit: 100,000,000 characters.

💡 Quick Summary

Three to One converts three-letter amino acid codes (e.g. AlaCysAspGlu) to their single-letter equivalents (e.g. ACDE). Digits and whitespace in the sequence body are removed automatically before conversion. Non-standard triplets are passed through without conversion. Stop codons written as "***" are converted to "*".

📋 How to Use
  1. Paste a raw three-letter sequence or one or more FASTA sequences into the input area. Input limit is 100,000,000 characters.
  2. Click Convert. Each sequence is output as a FASTA entry with a header line that identifies the source sequence name and its first few characters.
  3. The Summary panel shows how many sequences were processed, how many residues were successfully converted, and how many unrecognised characters were passed through.
  4. Use the Copy button to copy the single-letter output to your clipboard.
  5. Click Load Example to try with sample sequences containing all 23 supported three-letter codes including ambiguous residues and a stop codon.
  6. Click Clear to reset.
🧮 Formulas & Logic
Case normalisation
Every group of three characters is normalised to Title Case (e.g. "ALA" or "ala" → "Ala") before lookup.
Triplet lookup
Each normalised three-letter code is replaced by its IUPAC single-letter equivalent (see table below).
Stop codon
"***" → "*"
Filtering
Digits and whitespace are stripped from the sequence body before conversion; non-letter characters (other than *) are retained if they do not form a recognised triplet.
📊 Result Interpretation
Sequences Processed

Number of FASTA entries (or bare sequences) successfully converted.

Residues Converted

Total number of recognised three-letter codes that were successfully replaced with a single-letter equivalent.

Unrecognised Characters

Characters remaining after conversion that did not match any known three-letter code. These pass through to the output unchanged.

🔬 Applications
  • Converting three-letter protein sequences from databases or publications into FASTA single-letter format for downstream tools
  • Reformatting manually annotated protein sequences for alignment or BLAST searches
  • Batch-converting multiple sequences copy-pasted from papers or lab notebooks
  • Verifying that a three-letter sequence encodes the expected single-letter sequence
⚠️ Common Mistakes & Warnings
Non-standard triplets are passed through

Any group of three characters that does not match a recognised amino acid code (after case normalisation) is left in the output as-is. This may cause unexpected characters in the single-letter output if the input contains non-standard codes.

Digits and whitespace are stripped first

All digits and whitespace (spaces, tabs, newlines) in the sequence body are removed before conversion. This allows sequences with embedded numbering or line-wrapped text to be handled automatically.

Case is normalised before lookup

The input is processed in groups of three characters, and each group is normalised to Title Case (first letter upper, next two lower) before the lookup table is consulted. This means "ALA", "ala", and "AlA" are all recognised as Alanine.

❓ Frequently Asked Questions

Which three-letter codes are supported?
All 20 standard amino acids are supported, plus the three IUPAC ambiguity codes: Asx (B — Asn or Asp), Glx (Z — Gln or Glu), and Xaa (X — unknown residue). Stop codons written as "***" are converted to "*".
What happens to codes the tool does not recognise?
Unrecognised groups of three characters are passed through to the output unchanged. The "Unrecognised Characters" stat in the Summary panel shows how many characters were not converted.
Does capitalisation matter?
No. Every group of three characters is normalised to Title Case before lookup, so "ALA", "ala", "aLa", and "Ala" all produce "A".
Can I process multiple sequences at once?
Yes. Paste any number of FASTA-formatted sequences (each starting with ">title") and all will be converted in a single run.
How do I represent a stop codon?
Write the stop codon as three asterisks: ***. It will be converted to the single-letter stop symbol "*".
Three to One — logic ported from the Sequence Manipulation Suite (Paul Stothard, University of Alberta).