TPMT*15 is assigned no function based on limited evidence. Erythrocytes from a patient genotyped TPMT*3A/*15 exhibited very low enzymatic activity (15083071, 31005613). The patient's mother was genotyped TPMT*1/*15 and exhibited intermediate enzymatic activity (15083071). The TPMT*15-defining SNV, c.495-1G>A, disrupts the canonical splice site and results in the loss of exon 7, which is further supported by SpliceAI predictions. Additionally, the variant effect predictors, CADD and PhD-SNPg, predict that c.495-1G>A impairs protein function.