A rare consequence of bone-marrow transplantation gave scientists the opportunity to observe the evolution of one cancer in two patients. Seven years after donating bone marrow and white blood cells called leukocytes to her sister, who was battling leukaemia, a woman was diagnosed with a cancer of the lymph nodes known as follicular lymphoma. Fewer than six months later, so was her sister.

David Weinstock at the Dana-Farber Cancer Institute in Boston, Massachusetts, and his colleagues found that the cells of both lymphomas looked identical. In addition to a few unique mutations in each lymphoma, the team also identified 14 mutations that were shared between the two cancers and a frozen sample of the leukocytes originally transplanted from the donor sister. Thus, both cancers originated in the donor sister, with some mutations occurring before transplantation and others developing afterwards.

Cancer Discov. 10.1158/2159-8290.CD-11-0208 (2011)