Oto-palato-digital syndrome is the generalised term for two conditions, oto-palato-digital syndrome type I (OPD1) and oto-palato-digital syndrome type II (OPD2), that are both X-linked recessive genetic disorders with overlapping phenotypes. The most severe phenotypes of each syndrome occur only in males, with females generally having attenuated forms of the condition, although this does not apply to all individual cases. Some writers conceptualise oto-palato-digital syndrome as a spectrum disorder including two similarly-presenting genetic syndromes, frontometaphyseal dysplasia and Melnick-Needles syndrome.