Sometimes one just isn't enough: do vertebrates contain an H2A.Z hyper-variant?

How much functional specialization can one component histone confer on a single nucleosome? The histone variant H2A.Z seems to be an extreme example. Genome-wide distribution maps show non-random (and evolutionarily conserved) patterns, with localized enrichment or depletion giving a tantalizing suggestion of function. Multiple post-translational modifications on the protein indicate further regulation. An additional layer of complexity has now been uncovered: the vertebrate form is actually encoded by two non-allelic genes that differ by expression pattern and three amino acids. See research articles http://www.biomedcentral.com/1741-7007/7/86 and http://www.biomedcentral.com/1471-2148/9/31.