Despite the complex regulation of the expression of these genes, some general conclusions seem to be warranted. First, nearly all the studied genes are regulated in a tissue-specific fashion. Moreover hepatic expression seems to be more tightly controlled than intestinal expression. In the liver apo A-I and apo A-IV gene expression is regulated by steroid and thyroid hormones, whereas intestinal apo A-I and apo A-IV gene expression is unaffected by the same factors. The total concentration of apo B mRNA in the liver remains relatively constant after treatment with different hormones, but the relative level of apo B-100 and apo B-48 mRNA has been shown to be strongly regulated by thyroid hormones (Davidson et al., 1988). Again this type of regulation is not observed in the intestine. These observations therefore might indicate that common factors may be involved in the tissue selective expression and regulation of the apolipoprotein genes. Second, hepatic apo A-I and apo A-II gene expression is regulated in an opposite direction after treatment with estrogens, thyroid hormones and corticosteroids. This may be important since both apolipoproteins may perform opposite functions in the process of reverse cholesterol transport (Barbaras et al., 1986; Barkia et al. 1991). Accordingly, the opposite regulation of the expression of these genes by hormones and other factors may have important implications for the ant-atherogenic efficiency of HDL particles. Again it would be worthwhile investigating whether a common transcriptional regulator is involved in this opposite regulation