Preaxial polydactyly II (PPD2) is a limb malformation in which duplication, full or partial, of the first digital ray of hands or feet results in extra digits. Triphalangeal thumb is characterized by the presence of 3 phalanges within the thumb. The extra middle phalanx may be fully formed, trapezoidal, or a small triangular 'delta' phalanx; the thumb may be opposable or nonopposable. Preaxial polydactyly and triphalangeal thumb may cosegregate, or each occur in isolation, within families with mutation in the zone of polarizing activity (ZPA) regulatory sequence (ZRS), a regulatory element for the SHH gene (600725) that is contained within intron 5 of the LMBR1 gene (Heutink et al., 1994; Furniss et al., 2008; VanderMeer et al., 2014). [from
OMIM]