CYP2D6*27 is assigned normal function based on limited evidence. Multiple in vitro studies found CYP2D6*27 demonstrated similar activity compared to wildtype with various substrates (18784265, 18838503). Although some in vitro studies found CYP2D6*27 demonstrated modestly decreased activity it was not convincing the magnitude of decreased activity was clinically significant (24647041, 30366777). Therefore, consensus among experts was normal function with an activity value of 1 based on limited evidence as it was determined the phenotypes containing this allele would be clinically similar to phenotypes comprising normal function alleles.