A compound which is a fused pyrimidine of formula (Ia): wherein: X is S; R 2 is a heteroaryl group selected from the following structures (a) to (j): wherein: each W is, independently, -CH or N; Y is N, O or S; and B is a 5-, 6- or 7-membered saturated or unsaturated carbocyclyl or heterocyclic ring, which group is unsubstituted or substituted by a group R 20 or by alkyl which is unsubstituted or substituted by a group R 20; R 20 is selected from hydrogen, halogen, alkoxy, carbocyclyl, a 5- or 6-membered saturated N-containing heterocyclic group as defined above, OH, SR, CN, nitro, NR 2, - C 10 cycloalkyl and m is 1 or 2; and R 3 is selected from: (a) a group of the following formula: wherein B is a phenyl ring which is unsubstituted or substituted and Z is selected from H, -OR, -SR, CH 2 OR, -CO 2 R, CF 2 OH, CH(CF 3)OH, C(CF 3) 2 OH, -(CH 2) q OR,-(CH 2) q NR 2, -C N 2, -NR 2, -NRC R, -S m N 2, -OC R, OC N 2,-NRS m R, -RC N 2, CN and -NO 2, wherein each R is independently selected from H, C 1 -C 6 alkyl, C 3 -C 10 cycloalkyl and a 5- to 12-membered aryl or heteroaryl group, the group being unsubstituted or substituted, m is 1 or 2 and q is 0, 1 or 2; (b) a heteroaryl group which contains 1, 2, 3 or 4 ring nitrogen atoms and 0, 1 or 2 additional heteroatoms selected from O and S, which group is monocyclic or bicyclic and which is unsubstituted or substituted; and (c) a group comprising a benzene ring which is unsubstituted or substituted and which is fused to a heteroaryl group as defined above; or a pharmaceutically acceptable salt thereof. X is S; R 2 is a heteroaryl group selected from the following structures (a) to (j): each W is, independently, -CH or N; Y is N, O or S; and B is a 5-, 6- or 7-membered saturated or unsaturated carbocyclyl or heterocyclic ring, which group is unsubstituted or substituted by a group R 20 or by alkyl which is unsubstituted or substituted by a group R 20; R 20 is selected from hydrogen, halogen, alkoxy, carbocyclyl, a 5- or 6-membered saturated N-containing heterocyclic group as defined above, OH, SR, CN, nitro, NR 2, - C 10 cycloalkyl and m is 1 or 2; and R 3 is selected from: (a) a group of the following formula: wherein B is a phenyl ring which is unsubstituted or substituted and Z is selected from H, -OR, -SR, CH 2 OR, -CO 2 R, CF 2 OH, CH(CF 3)OH, C(CF 3) 2 OH, -(CH 2) q OR,-(CH 2) q NR 2, -C N 2, -NR 2, -NRC R, -S m N 2, -OC R, OC N 2,-NRS m R, -RC N 2, CN and -NO 2, wherein each R is independently selected from H, C 1 -C 6 alkyl, C 3 -C 10 cycloalkyl and a 5- to 12-membered aryl or heteroaryl group, the group being unsubstituted or substituted, m is 1 or 2 and q is 0, 1 or 2; (b) a heteroaryl group which contains 1, 2, 3 or 4 ring nitrogen atoms and 0, 1 or 2 additional heteroatoms selected from O and S, which group is monocyclic or bicyclic and which is unsubstituted or substituted; and (c) a group comprising a benzene ring which is unsubstituted or substituted and which is fused to a heteroaryl group as defined above; (a) a group of the following formula: wherein B is a phenyl ring which is unsubstituted or substituted and Z is selected from H, -OR, -SR, CH 2 OR, -CO 2 R, CF 2 OH, CH(CF 3)OH, C(CF 3) 2 OH, -(CH 2) q OR,-(CH 2) q NR 2, -C N 2, -NR 2, -NRC R, -S m N 2, -OC R, OC N 2,-NRS m R, -RC N 2, CN and -NO 2, wherein each R is independently selected from H, C 1 -C 6 alkyl, C 3 -C 10 cycloalkyl and a 5- to 12-membered aryl or heteroaryl group, the group being unsubstituted or substituted, m is 1 or 2 and q is 0, 1 or 2; (b) a heteroaryl group which contains 1, 2, 3 or 4 ring nitrogen atoms and 0, 1 or 2 additional heteroatoms selected from O and S, which group is monocyclic or bicyclic and which is unsubstituted or substituted; and (c) a group comprising a benzene ring which is unsubstituted or substituted and which is fused to a heteroaryl group as defined above;