Various prenatal exposures, including misoprostol, thalidomide, ergotamine, cocaine, and maternal homocystinuria, have been associated with an increased risk for MBS.20, 21, 22, 23, 24 Rare individuals have been reported to harbor heterozygous de novo variants in PLXND1 (HGNC:9107, OMIM 604282), REV3L (HGNC:9968, OMIM 602776),25,26 and CHN1 (HGNC:1943, OMIM 118423),27 as well as complex rearrangements disrupting semaphorin-plexin genes.28 The gene discussed is REV3L; the disease is Mobius syndrome.