CYP2B6*28 (c.917C>G, p.T306S; c.1132C>T, p.R378X) – No function: CYP2B6*28 is assigned no function. CYP2B6*28 is defined by a nonsense mutation which results in a premature stop codon resulting in a truncated protein. One study evaluating efavirenz plasma concentrations in patients with HIV found a subject with CYP2B6 *6/*28 genotype had elevated efavirenz AUC compared to the *1 reference and CYP2B6 *6/*6 subjects (17235330). In vitro studies using cDNA expression systems found that CYP2B6*28 demonstrated no enzyme activity (17235330, 20517174). Therefore, consensus among experts was no function based on limited evidence.