In this book, the authors examine retinoic acid and how it plays important role in a wide range of biological processes including proliferation and differentiation especially epithelial tissues. The mechanism of actions of retinoic acid in human keratinocytes and dermal fibroblasts are reviewed, as well as its clinical use in dermatological treatments. Next, the control of gene expression in hepatocytes is discussed. Retinoic acids (RA), including all-trans and 9-cis retinoic acids, are the bioactive components that bind and activate their cognate nuclear receptors to regulate target genes. The authors conclude by testing whether increased progesterone can bias sex ratios in the domestic hen. They determine that there is not a significant shift of the sex ratio after these preliminary experiments. These results suggest the existence of progesterone as additional inducer of meiosis in embryonic germ cell and independently of RA signaling in the chicken.