A method of inhibiting steroid sulphatase activity in a subject in need of same as described. The method comprises administering to said subject a steroid sulphatase inhibiting amount of a ring system compound; which ring system compound comprises a ring to which is attached a sulphamate group of the formula wherein each of R1 and R2 is independently selected from H, alkyl, alkenyl, cycloalkyl and aryl, or together represent alkylene optionally containing one or more hetero atoms or groups in the alkylene chain; and wherein said compound is an inhibitor of an enzyme having steroid sulphatase activity (E.C.3.1.6.2); and if the sulphamate group of said compound is replaced with a sulphate group to form a sulphate compound and incubated with a steroid sulphatase enzyme (E.C.3.1.6.2) at a pH 7.4 and 37° C. it would provide a Km value of less than 50 &mgr;M.