CD81 and neoplasm: These findings are supported by several direct facts, including the following: (i) CD9P-1 expression correlated with the metastatic status of hLT and was inversely proportional to CD9 expression; (ii) the truncated form of CD9P-1, GS-168AT2, was rapidly internalised by tumour cells correlating with its interaction with both CD9 and CD81; and most importantly, (iii) treatments of hLT-xenografted Nude mice with GS-168AT2 led to a drastic inhibition of hLT growth that was associated with an important in vivo downregulation of CD9 in the tumour core, but not in CD81.