SUN2 (sad1/unc-84 protein-like 2), also known as UNC84B (unc-84 homolog B), FRIGG, KIAA0668 or RAB5IP, is a 717 amino acid single-pass membrane protein that contains one SUN domain and localizes to the membrane of both the nucleus and the endosome. Widely expressed in a variety of tissues, including lung, muscle and heart, SUN2 interacts with Rab 5A and may play a role in homotypic endosome fusion. The gene encoding SUN2 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.