The function of LYSET (lysosomal enzyme trafficking factor, Ensembl gene identifier ENSG00000153485) is as follows. Required for mannose-6-phosphate-dependent trafficking of lysosomal enzymes (PubMed:36074821, PubMed:36074822, PubMed:36096887). LYSET bridges GlcNAc-1-phosphate transferase (GNPTAB), to the membrane- bound transcription factor site-1 protease (MBTPS1), thus allowing proteolytic activation of the GNPTAB. GNPTAB is involved in the regulation of M6P-dependent Golgi-to-lysosome trafficking of lysosomal enzymes (PubMed:36074821, PubMed:36074822, PubMed:36096887). LYSET is thus an essential factor for maturation and delivery of lysosomal hydrolases (PubMed:36074822). {ECO:0000269|PubMed:36074821, ECO:0000269|PubMed:36074822, ECO:0000269|PubMed:36096887}. (Microbial infection) Essential for infection by muliple viruses, including SARS-CoV-2, that utilize activated cathepsins for entry after M6P-dependent lysosomal transport. .