DLL4 and Adams-Oliver syndrome: Six causative genes with variations have been identified in relation to AOS including ARHGAP31, DOCK6, EOGT, RBPJ, NOTCH1, and DLL4. In a four-generation family with ARHGAP31 variation, all cases with ARHGAP31 variations showed TTLD indicative of high penetrance, suggesting ARHGAP31 variation may only cause isolated phenotype of TTLD in AOS (Isrie et al., 2014).