The obtained results indicated that multidrug resistance in cancer cells was modulated by chelidonine and the Chelidonium majus extract, the effects being due to a down-regulation of ABC-transporter proteins (P-gp, MRP1 and BCRP), metabolic genes (GST, CYP3A4 and PXR) and an up-regulation of caspase family genes. Here, ABCG2 is linked to cancer.