A conjugate of formula (A): Wherein Y is selected from formulae A1 and A2: Z1 is a C1-3 alkylene group; Z2 is a C1-3 alkylene group; Q is: where QX is such that Q is an amino-acid residue, a dipeptide residue or a tripeptide residue; L is a linker connected to a cell binding agent; CBA is the cell binding agent; and n is an integer between 0 and 48.