Described herein are methods of expanding a population of hematopoietic stem cells by contacting the population of hematopoietic stem cells with an effective amount of an inhibitor of G-protein coupled receptor 65 (GPR65) and providing a population of expanded, substantially undifferentiated hematopoietic stem cells. Exemplary GPR65 inhibitors include siRNAs and certain sphingolipids. Also described are populations of expanded hematopoietic stem cells produced by the methods, media and kits containing GPR65 inhibitors, and methods for administering an expanded population of hematopoietic stem cells to patients.