TBC1D22A (TBC1 domain family, member 22A), also known as C22orf4, is a 517 amino acid protein that contains one Rab-GAP TBC domain and is thought to function as a GTPase-activating protein for Rab family members. Multiple isoforms of TBC1D22A exist due to alternative splicing events. The gene encoding TBC1D22A 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. Additionally, translocations between chromosomes 9 and 22 may lead to the formation of the Philadelphia chromosome and the subsequent production of the novel fusion protein BCR-Abl, a potent cell proliferation activator found in several types of leukemias.