Abstract
Oculofaciocardiodental (OFCD) syndrome is a rare X-linked dominant syndrome characterized by the involvement of the eyes, face, teeth, and heart with variable expressivity. The syndrome is caused by loss-of-function variants in the BCOR gene located on the X chromosome. OFCD affects only females with presumed embryonic lethality among males. We report a first case of a female with biallelic mosaic variants in BCOR gene, leading to a severe ocular phenotype including anterior segment dysgenesis, cataracts, and retinal involvement. The unique condition of biallelic mosaic loss-of-function mutations leads to a variable expression of an allele with the pathogenic variant, independent of the X-Inactivation pattern. This novel mechanism of co-existent biallelic mosaicism should be suspected in unexplained severe cases of OFCD.