Abstract: The present invention provides an assay for a modulator of cell cycle progression, which comprises: a) bringing into contact a TBX2 protein, an ARF promoter TBX-2 binding region and a putative inhibitor compound under conditions where the TBX2 protein and the binding region, in the absence of inhibitor, are capable of forming a complex; and b) measuring the degree of inhibition of binding or transcriptional activation caused by said inhibitor compound. The invention further provides a method for the diagnosis or prognosis of breast cancer which method comprises determining the amount of TBX2 nucleic acid or protein in a cell which cell is, or is suspected to be, a breast tumour cell or metastatic cell from a breast tumour.