My laboratory originally showed that heart valves form from the cells that line the early heart in response to a local signal from the muscle. We have continued to explore this process and we have identified several molecules that regulate this process. We found that exposure of valve-forming endothelial cells to TCE blocks a key process of cell shape change but cells that had already undergone the change in shape were comparatively less affected by the exposure. This argues that an early step in heart valve formation is likely to be responsible for some of the birth defects associated with exposure to TCE.