{{Rsnum
|rsid=67666821
|Gene=CYP3A4
|Chromosome=7
|position=99758183
|Orientation=plus
|Summary=CYP3A4 variant affecting acetaminophen, codeine and other medicines
|geno1=(-;-)
|geno2=(-;T)
|geno3=(T;T)
|Assembly=GRCh38
|GenomeBuild=38.1
|dbSNPBuild=141
|Gene_s=CYP3A4
}}[[rs67666821]] is a SNP describing the CYP3A4*20 allele.

The normal/common form for this SNP is actually the null (ie deleted) form; the very rare (< 0.06% frequency in Caucasians) form encoding a nonfunctional [[CYP3A4]] protein has a T (in dbSNP orientation) at this location. As of 2006, it was the only CYP3A4 SNP with a known functional consequence.{{PMID|16580902}}.

Carriers of one [[rs67666821]](T) allele have an intermediate CYP3A4 metabolizer phenotype, and might be susceptible to side effects during drug therapy with substrates or inhibitors of CYP3A4 such as [[acetaminophen]], [[codeine]], [[cyclosporin A]], [[diazepam]] and [[erythromycin]].