The high content of histone H3.3, a transcription-related histone variant, may account for the phenomenon of epigenetic memory, mentioned above, in which somatic nuclei transplanted to Xenopus eggs resist the switching off of genes active in donor cells [23]. For example, muscle-specific genes are actively transcribed in the nuclei of muscle cells. The unusually high H3.3 content in eggs may promote the continuing transcription of such genes in developing embryos in non-muscle cells, in a way that would not happen in sperm after fertilisation, because sperm nuclei do not have active muscle genes.
