Fused pyrimidines of formula (I): wherein R1-R3, A and n have any of the values described in the specification; and pharmaceutically acceptable salts thereof, have activity as inhibitors of PI3K and may thus be used to treat diseases and disorders arising from abnormal cell growth, function or behaviour associated with PI3 kinase such as cancer, immune disorders, cardiovascular disease, viral infection, inflammation, metabolism/endocrine disorders and neurological disorders. Processes for synthesizing the compounds are also described.