Reflex Sanger Sequencing Reveals a Rare UGT1A1 *28/*37 Compound Heterozygous Genotype After an Indeterminate Commercial Result in Late-Line Gastric Cancer

Background/Objectives: Irinotecan is a key late-line agent in metastatic gastric cancer, but its dose-limiting toxicities—severe neutropenia and diarrhea—are strongly influenced by UGT1A1 promoter polymorphisms. Commercial genotyping kits in Japan routinely interrogate only the common *6 and *28 alleles and may fail to resolve rarer TATA-box variants. We report a case in which an indeterminate commercial result unmasked an extremely rare loss-of-function genotype. Methods: A Japanese man in his 60s with HER2-negative, microsatellite-stable metastatic gastric cancer progressing after four prior lines underwent pretreatment UGT1A1 genotyping with a commercial DNA chip in anticipation of irinotecan. When the *28 result returned as indeterminate owing to a TA-ratio error, reflex Sanger sequencing of the UGT1A1 promoter TATA box was performed. Results: The chip reported *6 as wild-type but could not resolve *28. Sanger sequencing identified one allele with seven TA repeats (*28) and one with eight TA repeats (*37), with no wild-type six-repeat allele—a *28/*37 compound heterozygote. The *37 allele is exceptionally rare in Japanese individuals (ToMMo 61KJPN allele frequency 0.000294) and has historically been reported almost exclusively in individuals of African ancestry. This genotype has previously caused fatal neutropenic colitis despite preemptive dose reduction. Given the patient’s frailty and limited expected benefit, irinotecan was withheld entirely. Conclusions: Indeterminate commercial UGT1A1 results may conceal rare loss-of-function alleles with divergent functional consequences. Such results should trigger confirmatory sequencing rather than empiric dose reduction, and institutions should establish reflex pathways in advance to support biomarker-informed, frailty-adapted treatment selection.