GO:0036351

The modification of histone H2A by addition of ubiquitin group at lysine 13 (H2A-K13) in metazoans, and at the equivalent residue in other organisms. Monoubiquitin is first attached to H2A-K13 and K63-linked ubiquitin chains are then extended from this monoubiquitin. [GOC:sp, PMID:22713238, PMID:22980979]