SPATA18 is a 538 amino acid protein that is thought to play a role in cell differentiation during spermatogenesis, particularly during development from late elongate spematids to mature spermatozoa. Localizing to cytoplasm, SPATA18 is encoded by a gene that maps to human chromosome 4q12. 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 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.