SLCO1B1*31 is assigned no function based on moderate evidence. SLCO1B1*31 is defined by c.388A>G (p.N130D, defining *37) and c.1463G>C (p.G488A, defining *9). SLCO1B1*37 is assigned normal function based on strong evidence as c.388A>G was not associated with altered clearance of SLCO1B1-transported drugs. SLCO1B1*9 is assigned no function based on moderate in vitro and clinical evidence. An individual genotyped SLCO1B1*1/*31 exhibited markedly decreased high-dose methotrexate clearance compared to the reference allele, and clearance was decreased compared to a SLCO1B1*5/*5 patient (22147369). An individual genotyped SLCO1B1*31/*41 exhibited an elevated plasma concentration of coproporphyrin I, at a level comparable to SLCO1B1*15/*15 patients, and this individual experienced statin-induced rhabdomyolysis (39511784). In vitro uptake of methotrexate by protein encoded by SLCO1B1*31 was significantly decreased compared to the reference; transport activity was similar to empty vector controls (22147369). Ten out of ten variant effect predictors (VEPs) tested by CPIC staff predict that c.388A>G is neutral for protein function. Nine out of ten VEPs predict that c.1463G>C impairs protein function.