Hollow mesoporous silica (HMS) spheres preloaded with the anticancer drug Doxorubicin (DOX) can be synthesized by using porous CaCO3 spheres as templates (see figure). With good biocompatibility, the HMS@DOX spheres exhibit a greatly enhanced therapeutic effect towards tumor cells; this enhancement may be caused by the efficient protection of DOX and the pH-sensitive release of DOX, together with enhanced intracellular delivery.