The basic helix-loop-helix (bHLH) transcription factor Scl/Tal1, hereafter referred to as Tal1, an oncogene originally identified in childhood leukemias, is expressed in endothelial, endocardial and hematopoietic, but not myocardial cells during early murine development [13]. The gene discussed is TAL1; the disease is leukemia.