To define the developmental origin and dynamics of the tumour infiltrating αβILTCKs, the authors utilised tumour bearing Fgd5-creER-Rosa26LSL-tdTomato PyMT mice, in which a pulse of tamoxifen injection stably labels Lin−c-KIT+SCA1+ haematopoietic stem cells to allow fate-mapping of their progenitors. The gene discussed is FGD5; the disease is neoplasm.