A thienopyrimidine of formula (I): and the pharmaceutically acceptable salts thereof have activity as inhibitors of PI3K with selectivity for the P110α subtype, and may be used to treat diseases and disorders arising from abnormal cell growth, function or behaviour, particularly those 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.