Through homozygosity-by-descent mapping and whole exome sequencing, and using an iterative analytic process, we identified two individuals from separate affected branches with a known mutation in the polymicrogyria gene, ADGRG1, and an early truncating mutation in a new candidate gene, retinol dehydrogenase 14 (RDH14), in the two remaining ID members. Here, ADGRG1 is linked to polymicrogyria.