MFAP3L (microfibrillar-associated protein 3-like), also known as HSD39 or testis development protein NYD-SP9, is a 409 amino acid single-pass type I cell membrane protein that contains one Ig-like (immunoglobulin-like) domain. Found primarily in testis, MFAP3L is encoded by a gene that is located on chromosome 4 and is expressed as three isoforms due to alternative splicing events. Representing approximately 6% of the human genome, chromosome 4 contains nearly 900 genes, one of which is the Huntingtin gene, which is found to encode an expanded glutamine tract in cases of Huntington's disease.