a 2015 review of polymorphisms in "abcb1" found that "the effect of "abcb1" variation on p-glycoprotein expression (messenger rna and protein expression) and/or activity in various tissues (e.g. the liver, gut and heart) appears to be small. although polymorphisms and haplotypes of "abcb1" have been associated with alterations in drug disposition and drug response, including adverse events with various abcb1 substrates in different ethnic populations, the results have been majorly conflicting, with limited clinical relevance."