CYP2D6 has received the most attention:

• It is estimated that about 25% of common drugs are metabolized by CYP2D6.
• CYP2D6 accounts for only about 1% of all CYP450 enzymes, but it is important in the metabolism of about 100 drugs.
• There are more than 80 genetic variants that have been described in the CYP2D6 gene. The normal, wild-type allele displays normal metabolic activity whereas some of the variant forms have enhanced or diminished activity.
• The variants can be grouped generally according to the resulting alterations in protein function.
• The groupings correlate with four major enzyme metabolic capacities (phenotypes): poor, intermediate, extensive (normal), or ultra-rapid metabolizers.