We have identified and characterized the novel human transmembrane protein 9 (TMEM9). TMEM9 encodes a 183 amino-acid protein that contains an N-terminal signal peptide, a single transmembrane region, three potential N-glycosylation sites, and three conserved cys-rich domains in the N-terminus, but no hitherto known functional domains. The protein is highly conserved between species from Caenorhabditis elegans to man and belongs to a novel family of transmembrane proteins. The TMEM9 gene consists of at least 6 exons and is localized to chromosome 1q41. TMEM9 mRNA is expressed in a wide range of tissues and cells. COS-1 cells transfected with a TMEM9 expression plasmid gave three bands of about 28, 3 1, and 33 kDa representing glycosylated forms of TMEM9 with a protein backbone of about 26 kDa. In COS-1 cells transfected with a TMEM9-GFP expression construct TMEM9-GFP is co-expressed with LAMP1 on late endosomes and lysosomes as well as on ER. Thus, TMEM9 is a phylogenetically conserved, widely expressed transmembrane protein with a potential, but unknown function in intracellular transport. (C) 2002 Elsevier Science (USA). All rights reserved.