Abstract: A compound of formula (I): for use in treating cancer or other diseases ameliorated by the inhibition of PARP, wherein: A and B together represent an optionally substituted, fused aromatic ring; X can be NRX or CRXRY; if X?NRX then n is 1 or 2 and if X?CRXRY then n is 1; RX is selected from the group consisting of H, optionally substituted C1-20 alkyl, C5-20 aryl, C3-20 heterocyclyl, amido, thioamido, ester, acyl, and sulfonyl groups; RY is selected from H, hydroxy, amino; or RX and RY may together form a spiro-C3-7 cycloalkyl or heterocyclyl group; RC1 and RC2 are independently selected from the group consisting of hydrogen and C1-4 alkyl, or when X is CRXRY, RC1, RC1, RC2, RX and RY, together with the carbon atoms to which they are attached, may form an optionally substituted fused aromatic ring; R1 is selected from H and halo; and Het is selected from: where Y1 is selected from CH and N, Y2 is selected from CH and N, Y3 is selected from CH, CF and N, where only one or two of Y1, Y2 and Y3 can be N