The present invention is based upon the finding that the protein p300 has acetylation activity which is directed to the retinoblastoma tumour suppressor protein pRb by the presence in the cell of the adenovirus E1A protein. This represents a target for modulators of the cell cycle, to which end the invention provides an assay for a modulator of acetylation of pRb by p300, which comprises: a) bringing into contact a p300 protein a pRb protein and a putative modulator compound under conditions where the p300 protein, in the absence of said modulator is capable of acetylating the pRb protein; b) providing conditions for acetylation of said pRb protein; and c) measuring the degree of inhibition of acetylation caused by said modulator compound.