C9orf4 is a 344 amino acid single-pass membrane protein that is primarily expressed in adult and fetal brain and is weakly expressed in spinal cord, adult ovary and medulla. C9orf4 contains one DOMON domain, a predominantly β-sheet domain that is thought to aide in extracellular adhesion. The gene encoding C9orf4 maps to human chromosome 9, which consists of about 145 million bases and 4% of the human genome and encodes nearly 900 genes. Considered to play a role in gender determination, deletion of the distal portion of 9p can lead to development of male to female sex reversal, the phenotype of a female with a male X,Y genotype. Hereditary hemorrhagic telangiectasia, which is characterized by harmful vascular defects, is associated with the chromosome 9 gene encoding endoglin protein, ENG. Familial dysautonomia is also associated with chromosome 9 though through the gene IKBKAP. Notably, chromosome 9 encompasses the largest interferon family gene cluster. Chromosome 9 is partnered with chromosome 22 in the translocation leading to the aberrant production of BCR-ABL fusion protein often found in leukemias.