There is provided an oligopeptide compound comprising: (a) at least one nitrogen-containing basic group attached to at least one end of the oligopeptide; and (b) two or more heterocyclic monomers, at least one of which is substituted in the heterocyclic part by a branched, cyclic or part cyclic C
3-5
alkyl group, or a pharmaceutically acceptable salt or solvate thereof; which compound, salt or solvate binds to the minor groove of DNA.