Although long-range chromatin organization during cell differentiation has not yet been determined, considerable evidence suggests that regulation of the chromatin structure may play a crucial part in transcriptional regulation. Drosophila genetics has introduced a unique system that can maintain genes in the on or off state after the initial gene expression decision has been established. This maintenance system is known to be mediated through the trithorax-group (trxG) and Polycomb-group (PcG) genes. The products from these 2 genes individually form multimeric complexes in the chromatin. The trxG genes are known to maintain the transcriptionally active states of the chromatin, and the PcG genes are thought to maintain the repressive states. The function of the PcG genes is defined in terms of anteroposterior patterning not only in Drosophila but also in mammals, whereas mammalian PcG genes have additional functions in higher order biological functions All PcG gene-deficient mice have provided evidence that these genes play a crucial role in hematopoiesis These findings should help shed new light on the roles of the chromatin regulatory system in hematopoiesis.