Cerebrotendinous xanthomatosis (CTX) is an inborn error of metabolism the hallmark features of which include chronic diarrhea, juvenile-onset cataracts, tendinous xanthomas, and progressive neurological dysfunction (OMIM 213700). CTX is an autosomal recessive genetic condition caused by biallelic variants in CYP27A1, which codes for the sterol 27-hydroxylase enzyme that functions in bile acid synthesis. Bi-allelic pathogenic variants in this gene result in decreased cholic acid and especially chenodeoxycholic acid synthesis and consequently cause elevations of cholestanol, bile acid intermediates, and bile alcohols. Diagnosis can be made with clinical diagnostic testing of plasma cholestanol, bile acid intermediate and bile alcohol levels, urinary bile alcohol levels, and/or genetic testing of CYP27A1. Understanding of the genetics of CTX can help facilitate diagnosis. Here we discuss the medical genetics, population genetics, and genotype phenotype correlations of CYP27A1.