One of the surprises of the genomic era is the relatively small number of genes that are present in vertebrates but not in flies (D. melanogaster). Hence, the greater complexity of vertebrates cannot be attributed to an increase in gene number. Instead, the vertebrate genome, which is about 30-fold larger than the fly genome, contains more genetic regulatory information outside protein-coding genes. Perhaps in response to this expansion of the genome, another strategy was used to regulate chromatin: combinatorial diversity. Current evidence indicates that many vertebrate chromatin-regulatory complexes are assembled combinatorially (see ref. 6 for a review), thereby greatly expanding the potential for diverse gene-expression patterns compared with unicellular eukaryotes. Arguably, the greatest need for diverse patterns of gene expression occurs in the development and function of the brain, and it may be no accident that an extraordinary diversity of neural phenotypes is emerging from genetic studies of the subunits of chromatin remodellers in the nervous system (see ref. 7 for a review).
