This review describes the chromosomal abnormalities in T-cell acute lymphoblastic leukaemia (ALL) which result in the over-expression of the gene SCL, which encodes a helix-loop-helix transcription factor. Also described are how gene targeting studies have revealed a key role for SCL in normal haemopoiesis. Next, the BCR-ABL fusion protein, seen in chronic myeloid leukaemia (CML) and in some patients with ALL, is discussed. Finally, the involvement of members of the core-binding factor (CBF) gene family in leukaemogenesis are described. Members of this gene family are involved in the generation of fusion proteins as a result of t(8;21) and inv(16), the most common translocations associated with acute myeloid leukaemia (AML). They provide a useful model of the way in which aberrant transcriptional function, brought about through genetic alterations, can modify haemopoietic development.