Variant histone H2A which replaces conventional H2A in a subset of nucleosomes. Nucleosomes wrap and compact DNA into chromatin, limiting DNA accessibility to the cellular machineries which require DNA as a template. Histones thereby play a central role in transcription regulation, DNA repair, DNA replication and chromosomal stability. DNA accessibility is regulated via a complex set of post-translational modifications of histones, also called histone code, and nucleosome remodeling. Acts as a Polycomb group (PcG) protein required to maintain the transcriptionally repressive state of homeotic genes of the animal throughout development. Required for histone H3 'Lys-9' methylation and histone H4 'Lys-12' acetylation, two modifications that are essential for heterochromatin formation. Also involved in DNA double strand break (DSB) repair. Essential for early development.