L6, IL-TMP, and TM4SF5 are cell surface proteins predicted to have four tra
nsmembrane domains. Previous sequence analysis led to their assignment as m
embers of the tetraspanin superfamily. In this paper, we identify a new seq
uence (L6D) that is strikingly similar to L6, IL-TMP, and TM4SF5. Analyses
of these four sequences indicate that they are not significantly related to
genuine tetraspanins, but instead constitute their own L6 superfamily.