TPMT*3A is assigned no function based on definitive evidence. Erythrocytes from individuals heterozygous for TPMT*3A exhibited intermediate enzymatic activity, while erythrocytes from individuals homozygous for TPMT*3A or heterozygous for TPMT*3A in combination with another no function allele exhibited low enzymatic activity (9103127, 9246020, 9931346, 15226673, 23252716, 31005613). Patients carrying TPMT*3A alleles exhibited increased plasma concentrations of TGNs, decreased plasma concentrations of methylated metabolites, and tolerate lower doses of thiopurines (10580024, 11304783, 19057372, 23585236, 29387964, 33205673, 33928426). In vitro studies found that TPMT.3A protein expression and enzymatic activity is substantially decreased or undetectable relative to TPMT.1 in yeast cells (8644731, 9177237), mammalian cells (16220112,18708949, 8561894), and patient erythrocytes (9177237, 8644731, 8561894). Loss of TPMT.3A protein expression is attributed to rapid degradation by the proteasome (16220112, 9177237, 15967990). Seven and eight out of ten variant effect predictors tested by CPIC staff predict that the TPMT*3A-defining SNVs, c.460G>A (p.A154T) and c.719A>G (p.Y240C), impair protein function, respectively.