Is there any role for the use of cytochrome P450 2D6 (CYP2D6) testing for tamoxifen resistance in routine practice?  

This is a matter of some debate. The earlier retrospective studies did show some lack of tamoxifen benefit with certain homozygous genotypes of CYP2D6, but some later, larger trials did not bear this out. Thus, the matter remains unsettled in my opinion.