In the chicken oviduct the synthesis of the egg-white proteins ovalbumin, conalbumin, ovomucoid and lysozyme is controlled by the female sex steroids. The steroid-controlled rate of synthesis of these proteins is closely correlated to the cellular concentration of the egg-white protein mRNAs. The accumulation of these mRNAs is achieved by a decreased rate of degradation and an increased rate of synthesis. To understand the molecular mechanism that regulates the expression of this group of genes we have isolated the lysozyme and ovomucoid gene by gene-cloning. The structure and expression of the lysozyme gene has been studied.