While other factors such as genotyping comprehensiveness (i.e. genotype test not including relevant PM alleles) or the use of tumour tissue with loss-of-heterozygosity at the CYP2D6 locus [43] also contributed to discrepancies between study findings as demonstrated by a recent meta-analysis [44], the lack of incorporation of CYP2D6 inhibitor usage may have added further heterogeneity [42]. This evidence concerns the gene CYP2D6 and neoplasm.