Also known as 1461_1462insA, 25889_25890insA, and 488Frameshift.[PMID 16580902]
Nonfunctional (and quite rare) allele of CYP3A4.
Heterozygotes with one of CYP3A4*20 allele plus one functioning allele are intermediate metabolizers for drugs metabolized by CYP3A4, and can develop significantly higher plasma concentrations of such drugs.[PMID 16580902]
Needs dbSNP rs#.