IUPAC Nucleotide Ambiguity Codes: Full Table

5 min read · Updated June 10, 2026

Sooner or later you hit a sequence full of letters that aren't A, C, G or T — an R here, a Y there, a run of Ns. These are IUPAC ambiguity codes, a standard shorthand for 'one of several possible bases' at a position. This guide is a quick reference to what each one means and where they come from.

Why ambiguity codes exist

A single letter sometimes has to stand for more than one base. It might be a position that varies across a population (a SNP), an uncertain base call from sequencing, or a deliberately degenerate position in a primer or a binding-site consensus. Rather than guess, the IUPAC codes record exactly which bases are possible at that spot.

The two-base codes

R = A or G (puRine)
Y = C or T (pYrimidine)
S = G or C (Strong, three H-bonds)
W = A or T (Weak, two H-bonds)
K = G or T (Keto)
M = A or C (aMino)

The three- and four-base codes

B = C, G or T (not A)
D = A, G or T (not C)
H = A, C or T (not G)
V = A, C or G (not T)
N = A, C, G or T (aNy base)

Complements of ambiguity codes

When you take a reverse complement, each ambiguity code maps to another code, not just A↔T and G↔C. R (A/G) complements to Y (T/C), W and S are self-complementary, K complements to M, and the three-base codes pair up B↔V and D↔H, while N stays N. A reverse-complement tool has to handle the whole alphabet to be correct.

R ↔ Y, K ↔ M, B ↔ V, D ↔ H
S ↔ S, W ↔ W, N ↔ N (self-complementary)

Where you'll meet them

Degenerate PCR primers designed to amplify a family of related sequences.
Restriction enzyme recognition sites with variable positions (e.g. HinfI, G^ANTC).
Consensus/binding-site motifs written as a single degenerate string.
Sequencing reads, where N marks a base the basecaller couldn't determine.

Worked example: writing one degenerate primer for a peptide

Because most amino acids are encoded by more than one codon, you can sometimes compress a whole family of DNA sequences that all encode a given peptide into a single IUPAC-coded primer. Take the 4-residue peptide Met-Phe-Asp-Gly (M-F-D-G) and look up each amino acid's codon(s) in the standard genetic code.

Concatenating the results gives the degenerate primer 5'-ATGTTYGAYGGN-3' (12 nt). The two Y's each stand for 2 possible bases and the N for 4, so this one written sequence represents 2 × 2 × 4 = 16 distinct real 12-mers — and every one of those 16 translates back to exactly Met-Phe-Asp-Gly, because each ambiguity code here was built only from codons that are genuinely synonymous for that residue.

Met (M): only one codon, ATG — no ambiguity needed.
Phe (F): TTT or TTC — the third position is C or T, which is Y, giving TTY.
Asp (D): GAT or GAC — again C or T at the third position, giving GAY.
Gly (G): GGT, GGC, GGA or GGG — all four bases appear at the third position, which is N, giving GGN.

IUPAC nucleotide codes vs. amino acid codes — same letters, unrelated meanings

Nucleotide ambiguity codes and the standard single-letter amino acid alphabet were both defined by IUPAC, and they draw from the same 26 letters — which means every one of the 11 ambiguity codes below is also a valid, completely unrelated amino-acid letter.

In practice this means a garbled or misrouted sequence rarely gets caught automatically: a degenerate DNA primer full of R's, Y's and N's is also a 'legal' string of amino-acid letters, so pasting it into a tool expecting protein input doesn't error out — it just silently produces a meaningless result. Check which alphabet a sequence is actually written in before trusting what a tool does with it.

R — A or G in DNA/RNA; Arginine in a protein sequence
Y — C or T in DNA/RNA; Tyrosine in a protein sequence
S — G or C in DNA/RNA; Serine in a protein sequence
W — A or T in DNA/RNA; Tryptophan in a protein sequence
K — G or T in DNA/RNA; Lysine in a protein sequence
M — A or C in DNA/RNA; Methionine in a protein sequence
B — C, G or T in DNA/RNA; Asx (Asn or Asp) in a protein sequence
D — A, G or T in DNA/RNA; Aspartate in a protein sequence
H — A, C or T in DNA/RNA; Histidine in a protein sequence
V — A, C or G in DNA/RNA; Valine in a protein sequence
N — any base in DNA/RNA; Asparagine in a protein sequence

Common mistakes with ambiguity codes

Assuming any amino acid's codons can be compressed into one IUPAC-coded triplet the same way. Leucine has six codons split across two unrelated groups — TTA/TTG and CTT/CTC/CTA/CTG. Merging them position-by-position gives Y-T-N (YTN), which expands to 8 triplets (CTA, CTC, CTG, CTT, TTA, TTC, TTG, TTT) — but TTC and TTT are actually Phe codons, not Leu. Leucine, Arginine and Serine all split this way and need two separate degenerate oligos, not one.
Reading an ambiguity code inside a restriction site as 'this sequence or that sequence' rather than 'this one position varies.' HinfI's site G^ANTC is a single 5-base pattern where positions 1, 2, 4 and 5 are fixed and only position 3 varies — it matches four real sequences (GAATC, GACTC, GAGTC, GATTC) simultaneously, not two alternative whole sites.
Treating an N from a sequencing read as if it were a genuine mixed base with known proportions. It almost always just means 'the base caller couldn't tell' — feeding it into a GC%, Tm or translation calculation as though it were a real, quantifiable mixture (the way a deliberately synthesized degenerate primer position is) will silently distort the result.

Frequently asked questions

What does N mean in a DNA sequence?

N is the IUPAC code for any base — A, C, G or T. It often marks a position where the base is unknown (an ambiguous sequencing call) or deliberately variable (as in a degenerate primer).

What is the complement of R in a reverse complement?

R (A or G) complements to Y (T or C). Ambiguity codes have their own complements: K↔M, B↔V, D↔H, while S, W and N are self-complementary.

Can every amino acid's codons be written as one degenerate IUPAC codon?

No. Most can — Gly compresses cleanly to GGN and Phe to TTY, for example. But Leucine, Arginine and Serine each have six codons split across two unrelated groups (Leu's codons are TTA/TTG plus CTT/CTC/CTA/CTG), so merging them position-by-position into a single triplet like YTN also matches codons for a different amino acid (Phe). Those three need two separate degenerate primers, not one.

Are IUPAC nucleotide ambiguity codes the same as amino acid one-letter codes?

No, though they use the same letters. Every one of the 11 nucleotide ambiguity codes (R, Y, S, W, K, M, B, D, H, V, N) is also a standard amino-acid letter with a completely unrelated meaning — R is 'A or G' in a DNA sequence but Arginine in a protein sequence. Tools don't always flag the mismatch if a sequence lands in the wrong field, so check which alphabet is expected.

How many actual DNA sequences does a string of ambiguity codes represent?

It depends on how many ambiguous positions are strung together, since the choices at each position multiply. Each two-base code (R, Y, S, W, K, M) contributes a factor of 2 and each N a factor of 4 — a primer written as GGN represents 4 real sequences, while one written as TTYGAYGGN (two Y's and one N) represents 2 × 2 × 4 = 16.