CYP2D6 and hyperinsulinemic hypoglycemia, familial, 4: CYP2D6 deficiency has previously been shown to be associated with lower levels of both tamoxifen active metabolites [2, 12, 20, 27] and the correlation of plasma level variability with CYP2D6 genotype was estimated to be 39 % for (Z)-endoxifen and 9 % for 4-OH-Tam [2].