Thiazolopyrimidines of formula (I): wherein W represents a thiazole ring; R1 and R2 form, together with the N atom to which they are attached, a group of the following formula (IIa): in which A is a ring system; m is 0, 1 or 2; R3 is H or C1-C6 alkyl; and R4 is an indole group which is unsubstituted or substituted; and the pharmaceutically acceptable salts thereof are inhibitors of PI3K and are selective for the p110δ isoform, which is a class Ia PD kinase, over both other class Ia and class Ib kinases. The compounds may 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 function disorders and neurological disorders.