Holoprosencephaly (HPE) is a common human birth defect caused by failure to properly form the midline of the midface and/or forebrain, and is frequently associated with mutations in genes that encode sonic hedgehog (SHH) signalling pathway components. The wide spectrum of clinical defects suggests the existence of silent modifier genes that influence the phenotype. Zhang et al. show, using a mouse model lacking the SHH pathway regulator Cdo, that a related gene, Boc, modifies the HPE phenotype, providing a framework to identify additional modifiers of HPE.

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