A conjugate comprising a PBD dimer linked from the tether joining the two PBD units, wherein the linker is of formula (A1) : (Formula (A1)) wherein RN1 is selected from H and C1-4 alkyl L is a linker connected to a cell binding agent CBA is the cell binding agent n is an integer selected in the range of 0 to 48 Q is: (Formula (Q)), where Qx is such that Q is an amino-acid residue, a dipeptide residue or a tripeptide residue.
