The invention relates to substituted pyrazolo[3,4-d]pyrimidine derivatives of the Formula-(I), or pharmaceutically-acceptable salts thereof, which possess anti-proliferative activity such as anti-cancer activity and are accordingly useful in methods of treatment of the human or animal body. The invention also relates to processes for the manufacture of substituted pyrazolo[3,4-d]pyrimidine derivatives, to pharmaceutical compositions containing the compound and to its use in the manufacture of medicaments for the production of an anti-proliferative effect in a warm-blooded animal such as man.