Thyroid hormone is essential for normal brain development. The main focus of research in the laboratory is to understand the molecular mechanism(s) by which thyroid hormone affects brain development and brain function. We pursue this goal within the context of two large projects. First, we are cloning genes expressed in the fetal brain that are regulated by maternal thyroid hormone. These genes have led us to examine the effect of thyroid hormone on cortical cell proliferation, differentiation and fate specification. Second, we are examining the mechanisms by which thyroid hormone action can be disrupted by polychlorinated biphenyls (PCBs). These chemicals are structurally similar to thyroid hormone. PCBs are known to affect neural development and they can alter circulating levels of thyroid hormone.