The subfamily I member CHD1 was initially thought to be integral to transcriptional activity. The tandem chromodomain of human CHD was found to specifically recognize and bind to the trimethylated lysine residue at position 4 of histone H3 (H3K4me3; a hallmark of actively transcribed chromatin)62, mediating subsequent recruitment of post-transcriptional initiation and pre-messenger-RNA splicing factors63. However, this role in transcriptional activation is either not general or not conserved, because Chd1-mutant D. melanogaster zygotes are viable and display only a mild notched-wing phenotype. Instead, in D. melanogaster, CHD1 seems to have a more important role in gametogenesis and as a maternal product. Both Chd1-null male and female D. melanogaster are sterile64. In females, oogenesis depends on the presence of functional CHD1 (ref. 64) (Fig. 2). Closer examination reveals that Chd1-mutant females, when mated to wild-type males, lay fertilized eggs that die before hatching. Maternal CHD1 is required for the incorporation of H3.3 into the male pronucleus during decondensation after fertilization. Failure to incorporate H3.3 may render the paternal genome unable to participate in mitosis in the zygote, resulting in non-viable haploid
