Model of induction of hematopoietic differentiation by specific transcription factors. In this model, transcription factors are expressed at low levels in CD34⁺ stem cells,134 as are specific growth factor receptors. Under direction of signals that are as yet not defined, such as the influence of stromal interactions or growth factor signalling, specific transcription factors, such as GATA-1 or PU.1,44 are upregulated. Upregulation of specific transcription factors leads to their autoregulation and upregulation of specific growth factor receptors, resulting in increases in proliferation, differentiation, and suppression of apoptosis of specific lineages. Downregulation of specific factors (such as GATA-1 during myeloid development) may also play an important role.