In many species, a variable proportion of the sperm genome remains associated with histones. For instance, in human spermatozoa, about 15% of the genomic DNA is bound by histones (Gatewood et al. 1990). It has been postulated that these histone-containing regions encompass a specific subset of early embryonic expressed genes (Gatewood et al. 1987). However, it has recently been shown that in human sperm, regions containing the genes of the two protamines and the transition protein TP2 are also enriched in histones (Wykes and Krawetz 2003). The labeling of specific gene-containing regions could not be the only function of the histone-containing genomic domains, since in both human and mouse spermatozoa, LINE/L1 elements (Pittoggi et al. 1999), as well as telomeric sequences (Zalenskaya et al. 2000), were found associated with histones.

Among histone variants, only CENP-A has been reported to survive histone displacement in mammals, but its possible role in late spermiogenesis or in post-fertilization events is not known (Palmer et al. 1990).

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