The present invention relates to a pyrrolobenzodiazepines (PBD), in particular pyrrolobenzodiazepines having a linker group connected to a cell binding agent. Further or alternatively, the invention relates to conjugates of formula Ia, pharmaceutical compositions thereof, use thereof in the preparation of a medicament for the treatment of a proliferative disease (in particular breast cancer), and/or PBD compounds IIa, IIIa, IVa and/or Va.