The present invention relates to an improved assay for identifying compounds that may be of use in conjunction with cancer chemotherapeutic agents and anti-proliferative agents, to improve efficacy of such agents and/or render effective compounds with relatively little therapeutic activity. There is also provided a class of compounds of formula (I) and retinoids identified by said assay which may be used in a combination therapy, with current and novel agents, to treat cancers and other diseases associated with abnormal host cell proliferation, such as psoriasis.