CYP26B1 and neural tube defect: As we previously reported [33], in six retinoid metabolism genes we found 12 occurrences of PDRVs in NTD cases versus 0/225 in controls (P = 0.0046, two-sided Fisher’s exact test) (Fig. 2c) and PDRVs were predominantly in the CYP26B1 gene (9/12), which acts to degrade retinoic acid.