Monarch Disease Ontology identifier MONDO_0010669 (syndactyly type 8) is a rare, genetic, non-syndromic, congenital limb malformation characterized by unilateral or bilateral fusion of the fourth and fifth metacarpals with no other associated abnomalities. Patients present shortened fourth and fifth metacarpals with excessive separation between their distal ends, resulting in marked ulnar deviation of the little finger and an inability to bring the fifth finger in parallel with the other fingers. Also known as: FGF16 non-syndromic syndactyly, fusion of metacarpals 4 and 5, metacarpal 4-5 fusion, X-linked recessive, non-syndromic syndactyly caused by mutation in FGF16.