CD9 and cancer: Although some reports have described several members of the tetraspanin family (i.e., CD9, CD81, TSPAN8, CD51) associated with CSC properties and/or resistance to current therapies in various cancer models [38,39,40,41,42]; as far as we know, this is the first time that TSPAN1 has been shown to be involved in the cellular response to chemotherapy.