It is still largely unknown what causes the group of diseases called myeloproliferative neoplasms (MPNs), including some rare forms of leukemia, where stem cells that replenish the blood supply begin to grow or differentiate out of control. But, a recent study shows their incidence is five to seven times higher among first-degree relatives of individuals who have been diagnosed with such a disease, providing strong evidence for a genetic link. This analysis, led by researchers from NCI's Division of Cancer Epidemiology and Genetics and collaborators in Sweden