PHF21B (PHD finger protein 21B), also known as PHF4, is a 531 amino acid protein that contains one PHD-type zinc finger and is expressed as multiple alternatively spliced isoforms. The gene encoding PHF21B maps to human chromosome 22, which houses over 500 genes and is the second smallest human chromosome. Mutations in several of the genes that map to chromosome 22 are involved in the development of Phelan-McDermid syndrome, neurofibromatosis type 2, autism and schizophrenia.