Pyrimidine derivatives of formula (I) wherein: Qh.sub.1 and Q.sub.2 are independently selected from aryl or carbon linked heteroaryl optionally substituted as defined within; and one of Q.sub.1 and Q.sub.2 or both Q.sub.1 and Q.sub.2 is substituted on a ring carbon by one group selected from sulphamoyl, N--(C.sub.1-4alkyl)sulphamoyl (optionally substituted by halo or hydroxy), N,N-di-(C.sub.1-4alkyl)sulphamoyl (optionally substituted by halo or hydroxy), C.sub.1-4alkylsulphonyl (optionally substituted by halo or hydroxy) or a substituent of the formula (Ia) or (Ia'): wherein Q.sub.1, Q.sub.2, G, R.sup.1, Y, Z, Q.sub.3, n and m are as defined within; and pharmaceutically acceptable salts and in vivo hydrolysable esters thereof are described. Processes for their manufacture, pharmaceutical compositions and their use as cyclin-dependent serine/threonine kinase (CDK) inhibitors are also described ##STR00001##