To study the potential effect of reactivation of gene expression from the inactive X-chromosome on NDD-associated phenotypes, we employed fibroblasts from a female heterozygous carrier of a 4-bp deletion in the MID1 gene, causative for Opitz BBB/G Syndrome (OS)21–23, as well as fibroblasts from her affected male fetus. The gene discussed is MID1; the disease is Opitz G/BBB syndrome.