A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene and are the result of genetic differences.

In genetics, gene isoforms are mRNAs that are produced from the same locus but are different in their transcription start sites (TSSs), protein coding DNA sequences (CDSs) and/or untranslated regions (UTRs), potentially altering gene function.