CCDC8 and 3M syndrome: According to the molecular etiology, it is divided into three forms, namely, 3M syndrome-1 (OMIM #273750), 3M syndrome-2 (OMIM #612921), and 3M syndrome-3 (OMIM #614205), which are caused by biallelic loss-of-function variants in the cullin 7 (CUL7), obscurin-like 1 (OBSL1), and coiled-coil domain-containing protein 8 (CCDC8) genes, respectively (3).