[en] It is pointed out that over an experimentally important kinematic region lepton pair production arises from the annihilation of a quark and antiquark within the same hadron. This renders incorrect the conventional extraction of structure functions from data in this region. Instead it now becomes possible to obtain information on the correlation between a quark and antiquark within the same proton. This kinematic region of interest is carefully quantified and it is illustrated with a discussion of the validity of some recent structure function analyses. (author)