Lysosomes are a major site of intracellular acidic hydrolase-mediated proteolysis and cellular degradation. The export of low-molecular catabolic end products from the lysosomal lumen to the cytosol is facilitated by polytopic transmembrane proteins which mediate secondary active or passive transport. One of these proteins is MFSD1, a so far uncharacterized lysosomal 12 transmembrane domain-containing family member of the major facilitator superfamily. MFSD1 is, uncommon for a lysosomal membrane protein, not N-glycosylated. It contains a dileucine-based sorting motif that is essential for its transport to lysosomes. Lysosomes from mouse liver of MFSD1 knock-out mice were isolated and analyzed by TMT labelling, as well as whole liver protein extracts, both in comparison to wildtype controls.