GO:0036352

The modification of histone H2A by addition of ubiquitin group at lysine 15 (H2A-K15) in metazoans, and at the equivalent residue in other organisms. Monoubiquitin is first attached to H2A-K15 and K63-linked ubiquitin chains are then extended from this monoubiquitin. [GOC:sp, PMID:22713238, PMID:22980979]