The 1661G>C variation in CYP2D6. Involved in a number of decreased and non-functioning CYP2D6 variants. This SNP does not yet appear to be common in consumer tests.
[PMID 18698231] Polymorphisms affecting gene transcription and mRNA processing in pharmacogenetic candidate genes: detection through allelic expression imbalance in human target tissues.
[PMID 23133420] Pharmacogenomic Diversity among Brazilians: Influence of Ancestry, Self-Reported Color, and Geographical Origin.