PGP and neoplasm: To determine whether or not LysoTracker and MitoTracker Red are transport substrates of P‐gp, these two pairs of cognate tumour cell lines were incubated with 100 nM of each of the two fluorescent probes for 1 hr in the presence or absence of the potent P‐gp transport inhibitor tariquidar (100 nM) 40.