SLCO1B1*31 is assigned no function based on limited evidence in heterozygous patients and in vitro studies. A clinical study of 1 heterozygous carrier of *31 showed markedly decreased high-dose methotrexate clearance compared to wildtype, slower than a *5/*5 patient (22147369). In vitro uptake was severely diminished compared to wildtype (22147369). Computational activity predictions support the in vitro and in vivo data (22147369).