Compounds of formula (I) have HSP90 inhibitory activity: ring A is an aryl or heteroaryl ring or ring system; R1 is hydrogen, fluoro, chloro, bromo, or a radical of formula (1A): —X-Alk1-(Z)m-(Alk2)n-Q (IA) wherein X is a bond, —O—, —S—, —S(O)—, —SO2—, or —NH—, Z is —O—, —S—, —(C═O)—, —(C═S)—, —S(O)—, —SO2—, —NRA—, or, in either orientation —C(═O)O—, —C(═O)NRA—, —C(═S)NRA—, —SO2NRA—, —NRAC(═O)—, or —NRASO2— wherein RA is hydrogen or C1-C6 alkyl in which one or more hydrogens is optionally substituted by fluorine; Alk1 and Alk2 are optionally substituted divalent C1-C3 alkylene or C2-C3alkenylene radicals, m and n are independently 0 or 1, and Q is hydrogen or an optionally substituted carbocyclic or heterocyclic radical; R2 is cyano (—CN), fluoro, chloro, bromo, methyl, ethyl, —OH, —CH2OH, —C(═O)NH2, —C(═O)H, —C(═O)CH3, or —NH2; R3 and R4 are independently selected from hydrogen, fluoro, chloro, bromo, cyano (—CN), C1-C3alkyl optionally substituted with one or more fluorine substituents, C1-C3alkoxy optionally substituted with one or more fluorine substituents, —CH═CH2, —C≡CH, cyclopropyl and —NH2, or R3 and R4 together represent methylenedioxy (—OCH2O—) or ethylenedioxy (—OCH2CH2O—) in either of which one or more hydrogens are optionally replaced by fluorine; S1 is as defined in the description.