Images of zebrafish (Danio rerio) embryos illustrating the function of the transporter protein Abcb4 acting as effective chemical defense by keeping out chemical compounds from the embryos. When exposed to the red, fluorescent dye rhodamine B little dye accumulates in the cells and tissues of the fish embryos because Abcb4 prevents uptake of the dye as can be seen in the image above. When transporter function is disrupted by a chemical inhibitor considerably more dye accumulates in the embryo tissues – the embryo tissues appear brighter as the image below shows. Abcb4 acts as uptake barrier for a range of toxic chemicals and protects the fish embryos from their toxic impact. However, similar to cyclosporin A, which here was used as transporter inhibitor, also other chemical compounds, including man-made pollutants, inhibit Abcb4 function and thus cause increased sensitivity of fish embryos to toxicants.