polydactyly of a biphalangeal thumb (Monarch Disease Ontology entry MONDO_0008269) or PPD1 is the most common form of preaxial polydactyly of fingers, a limb malformation syndrome, that is characterized by the duplication of one or more skeletal components of a biphalangeal thumb. Hands are preferentially affected (in bilateral), and the right hand is more commonly involved than the left. Also known as: PPD1, polydactyly, preaxial type 1, preaxial polydactyly type 1.