PGP and amyotrophic lateral sclerosis: Concerning P-gp, a reduced activity or incorrect polarization was suggested to occur for all disease lines, except SOD4AV (ALS), as evidenced by a rhodamine 123 efflux ratio smaller than 1, when compared to efflux ratios of 2–4 obtained for the three healthy control lines (the effectiveness of P-gp efflux activity is measured through the efflux ratio of the basolateral-to-apical and apical-to-basolateral permeabilities, and for P-gp substrates, the obtained value should be greater than 1.0).