Researchers have long known that fat will move into the placesleft behind when blood-producing cells are destroyed by chemotherapy or irradiation. New work, however, suggests that there is more going on. “We were taught in med school that fat was a space filler in the marrow, inversely proportional to haematopoietic content,” says George Daley of Children’s Hospital Boston. “What we show is that the fat actively influences HSC [haematopoietic stem cell] activity.” More specifically, Daley’s team shows that discouraging fat production in the marrow actually helps HSCs engraft1. This has important implications for helping patients recover after bone marrow transplants.