Regulates cell fate specification in the postembryonic mesodermal M lineage and body size, probably by positively modulating BMP-like Sma/Mab signaling at the ligand-receptor level. Promotes ventral fate specification in the M lineage, probably by positively modulating lin-12/Notch signaling. Specifically localizes to the basolateral side of intestinal cells. Localizes to cytoplasmic vesicles in the M mesoblast. Expressed at 100-cell embryonic stage, in the pharynx, intestine, hypodermis and M lineage (M-1 to M-16 stages) at the L1 larval stage and in somatic gonad, vulva and rectal epithelium at the L4 larval stage. RNAi-mediated knockdown in a sma-9 (cc604) mutant background restores the production of the 2 M lineage-derived coelomocytes. Belongs to the tetraspanin (TM4SF) family.