Compounds of formula (I) are inhibitors of HSP90, and useful in the treatment of, for example, cancers: wherein R2 is a group of formula (IA): -(Ar1)m-(Alk1)p-(Z)r-(Alk2)s-Q  (IA) wherein in any compatible combination Ar1 is an optionally substituted aryl or heteroaryl radical, Alk1 and Alk2 are optionally substituted divalent C1-C3 alkylene or C2-C3 alkenylene radicals, m, p, r and s are independently 0 or 1, Z is —O—, —S—, —(C═O)—, —(C═S)—, —SO2—, —C(═O)O—, —C(═O)NRA—, —C(═S)NRA—, —SO2NRA—, —NRAC(═O)—, —NRASO2— or —NRA— wherein RA is hydrogen or C1-C6 alkyl, and Q is hydrogen or an optionally substituted carbocyclic or heterocyclic radical; R3 is hydrogen, an optional substituent, or an optionally substituted (C1-C6)alkyl, aryl or heteroaryl radical; and R4 is (i) hydrogen, a —CN group, a nitro group —NO2, or a —C(═NOH)(NH2) group, or (ii) an optionally substituted C1-C6alkyl, aryl, heterocyclic, aryl(C1-C6alkyl)-, or heterocyclic(C1-C6alkyl)- group, or (iii) a group of formula —C(═O)R5 wherein R5 is hydroxyl, optionally substituted C1-C6alkyl, C1-C6alkyoxy, aryl, aryloxy, heteroaryl, heteroaryloxy, aryl(C1-C6alkyl)-, aryl(C1-C6alkoxy)-, heteroaryl(C1-C6alkyl)-, or heteroaryl(C1-C6alkoxy)-, or (iv) a group of formula —C(═O)NHR6 wherein R6 is primary, secondary, tertiary or cyclic amino, or hydroxyl, optionally substituted C1-C6alkyl, C1-C6alkyoxy, aryl, aryloxy, heteroaryl, heteroaryloxy, aryl(C1-C6alkyl)-, aryl(C1-C6alkoxy)-, heteroaryl(C1-C6alkyl)-, or heteroaryl(C1-C6alkoxy)-.