Figure 1 (A) Retinoic acid (RA) bioactivity in patient tissues of normal oesophagus with squamous epithelium (SE) and Barrett's metaplasia (Barrett's oesophagus (BE)) in the presence of the substrate retinol (10⁻⁶ M). Bioactivity in the medium was measured using a luciferase reporter assay. The scatter plot demonstrates the mean value for each patient and the overall mean. RA bioactivity is highest in Barrett's tissues (***, p<0.001). (B) Immortalised Barrett's cell lines (CP‐C and CP‐D) were treated with 10⁻⁷ M RA or 15 µM lithocholic acid (LCA) or a combination of both for 1 h and SYBR green‐based real‐time PCR was performed for the RA downstream gene, p21^WAF1. Increased p21 expression was observed in both cell lines treated by exogenous RA and LCA, which was synergistic (*p<0.001 in comparison to either LCA or all‐trans retinoic acid alone).