Compounds of the formula (I) wherein X represents hydrogen or hydroxy; R?1 and R2¿, which may be the same or different, represent hydrogen, (C¿1?-C4)alkyl optionally substituted with one hydroxyl group or one or more fluorine atoms, or, together with the carbon atom to which they are attached, R?1 and R2¿ form a (C¿3?-C5)carbocyclic ring; R?3¿ represents (C¿1?-C4)alkyl, (C1-C4)alkoxy or a halogen atom, such as fluorine, chlorine, bromine, or iodine, and in-vivo hydrolyzable esters thereof with pharmaceutically acceptable acids, may be used in the prophylaxis and/or treatment of diseases characterized by abnormal cell differentiation and/or cell proliferation.