ABCB1 and hepatitis B virus infection: Univariate linear regression analysis identified the following variables as predictors of efavirenz plasma level at week 4; CYP2B6*6 genotype (9.8%, p<0.0001), country (4.8%, p<0.0001), co infection with Hepatitis B at recruitment (0.6%, p = 0.061), CYP3A5*1 genotype (1.5%, p = 0.019), ABCB1 c.3435C>T (0.8%, p = 0.082), ABCB1 c.4036A>G (2.4%, p = 0.003), SLCO1B1*1b (0.8%, p = 0.084).