Mepacrine and LysoTracker accumulate in the same compartments in MEG-01 and undifferentiated G1ME2 MK-erythroid progenitors. MEG-01 cells (A,C-D) or G1ME2 MK-erythroid progenitor cells (G1ME2-MEP) (B,C-D) were incubated with 50 μM mepacrine (green) and 200 nM LysoTracker Red DND-99 (red) for 30 min. Images are of a single plane and not deconvolved. Scale bar, 5 μm; ×2 magnifications of boxed regions (insets); examples of overlap (arrows). (C) Quantification of the number (mean ± SD) of puncta per platelet labeled by mepacrine alone (# mep-only), LysoTracker Red alone (# LyTr-only), or both (# coloc.) in 30 cells of each type. (D) Quantification of the percentage (mean ± SD) of mepacrine labeling that colocalized with LysoTracker in MEG-01 (n = 37) and G1ME2-MEP (n = 43) cells, normalized to values for colocalization of LysoTracker Red with LysoTracker Green DND-26 in G1ME2-MK (N = 79), as in Figure 4. MEG-01 cells and G1ME2-MEPs were imaged by spinning-disk confocal microscopy at room temperature with an Ultraview inverted microscope equipped with a 63× Plan Apochromat lens, a Hamamatsu Orca-ER CCD camera, and Volocity software. Imaging medium was IMDM. Mepacrine has an excitation/emission of 436/525 nm; LysoTracker Red excitation/emission: 577/590 nm.