C4orf12 (chromosome 4 open reading frame 12) is a 926 amino acid single-pass membrane protein that exists as two isoforms. The gene encoding C4orf21 maps to human chromosome 4. Human chromosome 4 represents approximately 6% of the human genome and contains nearly 900 genes. Notably, the Huntingtin gene, which is found to encode an expanded glutamine tract in cases of Huntington's disease, is on chromosome 4. FGFR-3 is also encoded by a gene that is located on chromosome 4, and has been associated with thanatophoric dwarfism, achondroplasia, Muenke syndrome and bladder cancer. Chromosome 4 is also tied to Ellis-van Creveld syndrome, methylmalonic acidemia and polycystic kidney disease.