A crystalline form of (S)-1-(4-Chloro-phenyl)-7-isopropoxy-6-methoxy-2-(4-{methyl-[4-(4- methyl-3-oxo-piperazin-1-yl)-trans-cyclohexylmethyl]-amino}-phenyl)-1,4dihydro-2H- isoquinolin-3-one, which is useful in the treatment of a disease or disorder associated with the interaction between p53, or variants thereof, and MDM2 and/or MDM4, or variants thereof, respectively, (I).