SNP, Mutation, or Star Allele? The Language of Pharmacogenomics (PGx)

If you've ever read a pharmacogenomic (PGx) report and wondered why one finding appears as "CYP2C19 *2" in one place, "rs4244285" in another and "c.681G>A" in a third, you're not alone. Three terms come up constantly: SNP, mutation, and star allele. They overlap, they're often used interchangeably, and they describe genetic information at different levels.

What is a SNP?

A single nucleotide polymorphism (SNP) is a spot in the genome where a single DNA letter differs between people. At one position, most people might have a G while others have an A.

Two things define a SNP. First, it involves one letter only: a swap, not an insertion, deletion or large rearrangement. Second, it is common. By convention, the less frequent version must appear in at least about 1% of a population.

SNPs are catalogued in the NCBI database dbSNP with rsIDs (for example, rs4244285). An rsID points to a position and its alternatives. On its own it says nothing about function.

What is a mutation?

A mutation is any change in DNA compared with a reference. The word describes the event of change, with no limit on size or frequency. It could be a one-letter swap, the loss of thousands of letters, or a duplicated gene.

The word also carries baggage. In everyday speech, "mutation" suggests something rare and harmful, like a cancer-driving change. For this reason, professional guidelines such as the ACMG/AMP recommendations prefer the neutral term variant. In PGx, most variants are neither good nor bad. They matter only when a drug is involved.

This is where confusion begins. A change like CYP2C19 c.681G>A is common, so it counts as a SNP. It also reduces enzyme activity, so people casually call it a "mutation." Both labels fit, but each suggests something different.

What is a haplotype?

A haplotype is a group of variants that sit close together on the same copy of a chromosome and are usually inherited as a unit. You have two copies of most genes, one from each parent, so you carry two haplotypes for each gene. The key idea is that it is not enough to know which variants a person has. It also matters whether they are on the same copy or on different copies. For example, two SNPs on one copy of TPMT form the 3A haplotype, but the same two SNPs split across the two copies would be reported as different alleles (3B on one, *3C on the other) and are interpreted differently. Star alleles are simply haplotypes with official names.

What is a star allele?

A star allele is a naming system for pharmacogenes such as CYP2D6, CYP2C19, CYP2C9 and TPMT. It is written as the gene name, an asterisk and a number, like CYP2C19*2. The names are curated by the Pharmacogene Variation Consortium (PharmVar).

A star allele is a haplotype: the set of variants found together on one copy of a chromosome. It is not a single letter change. Key points:

• 1 is the reference allele, the default. It is often called "normal," but that is an assumption. If a test finds none of the variants it looks for, the result defaults to *1.

• Each other number defines a variant or a combination of variants. One star allele may rest on a single SNP, several SNPs, or a structural change.

• Two alleles make a diplotype, one from each parent, such as 1/2.

• Function is assigned afterward. 2 may be "no function," 17 "increased function," and the diplotype is then translated into a phenotype such as "poor metabolizer."

Where they overlap

• Star alleles are built from SNPs and other variants. CYP2C19*2 comes from one SNP (rs4244285) that disrupts the enzyme. CYP2C19*3 comes from a different SNP (rs4986893).

• Star alleles can include non-SNP changes. CYP2D6*5 is a whole-gene deletion and CYP2D6*1xN is a gene duplication.

• Every SNP is a variant, but not every variant is a SNP. A rare single-letter change is a single-nucleotide variant, but it falls short of the frequency rule.

• Whatever the naming system, the aim is to predict enzyme activity and guide drug choice or dose.

Why they're so easy to confuse

One variant, many names. The same CYP2C19 change can be called rs4244285, c.681G>A, or *2, depending on the database, paper or lab. Each is correct, but they come from different systems, and reports rarely explain the link.

A star allele can equal one SNP, which hides the difference. When 2 rests on a single SNP, people treat "2" and "rs4244285" as synonyms. That breaks with TPMT*3A, which needs two SNPs together on the same chromosome. Its parts, 3B and 3C, each carry only one.

*1 is a default, not a finding. Many people read *1/*1 as "tested and normal." Often it means "none of the variants on this panel were detected." A rare, untested variant could still be there.

"Mutation" is a loaded word. Patients may read it as disease risk. In PGx it usually just means the body handles a drug differently.

Genes follow different conventions. HLA-B*57:01 uses a colon format from protein-level typing, and DPYD results are often reported with HGVS names rather than star alleles.

Updates add noise. CYP2D6*4 has many suballeles (*4.001, *4.002 and so on), and definitions change as evidence grows.

Panels differ. Two labs can report different star alleles for the same person because they test different variants. A genotype is only as complete as the assay behind it.

Hope this solves the confusion a bit!

-Written by Sohni Tagore