Modifications leading to ineffectiveness of carbapenems in P. aeruginosa chronically established in the CF lung consist mainly in the loss (or the expression of an inactive form) of the OprD outer-membrane porin, which is responsible for the uptake of basic amino acids (notably arginine) and carbapenem drugs into the bacterial cell, followed by the overexpression of efflux pumps, mainly MexAB-OprM (limited to meropenem and doripenem). The gene discussed is OPRD1; the disease is cystic fibrosis.