Square planar complexes of type [MA2B2] show cis-trans isomerism. Cisplatin cis-[Pt(NH3)2Cl2] is a landmark anti-cancer drug; its trans isomer has no therapeutic activity. The cis form allows both Cl- to be replaced by DNA bases on the same strand, forming intrastrand crosslinks that inhibit replication. [MA2BC] type does not show geometrical isomerism. [MABCD] type shows 3 geometrical isomers. For [MA2B2] in square planar: cis has identical ligands adjacent (90 degrees), trans has them opposite (180 degrees).