CYP2B6*26 (c.499C>G, p.P167A; c.516G>T, p.Q172H/splice defect; c.785A>G, p.K262R) – Decreased function: CYP2B6*26 is assigned decreased function. CYP2B6*26 contains c.516G>T which causes aberrant splicing (see CYP2B6*6 for more information). One study evaluating efavirenz plasma concentrations among patients with HIV found CYP2B6*26 was associated with elevated efavirenz concentrations compared to the *1 reference and has similar function to CYP2B6*6. This was demonstrated by similar efavirenz concentrations among CYP2B6 *6/*26 subjects CYP2B6 *6/*6 subjects (17918089). Although one in vitro study using a cDNA expression system found that CYP2B6*26 demonstrated increased enzyme activity compared to the *1 reference (30201214), the experts did not find these results convincing as it has previously been shown that in vitro expression systems may not adequately reflect CYP2B6*6 expression and function, and therefore also not be a suitable system to assess CYP2B6*26. As is the same for *6, the mechanism for decreased activity is aberrant splicing caused by c.516G>T, which leads to reduced amounts of protein and thus, activity (14551287, 18171905, 24260284). As a result, alleles having c.516G>T are considered decreased function, regardless of the functional status of additional other variants unless these cause no function (e.g., CYP2B6*13 and CYP2B6*38). Therefore, the consensus among experts was decreased function based on the presence of c.516G>T and limited evidence.