The homeo box is a 180 base pair protein coding sequence that is a component of several Drosophila homeotic and segmentation genes. The subsequent identification of homeo box sequences in mammals raises the provocative possibility that at least some of the genes that control mammalian development are in hand. We have identified a new mouse homeo box gene called MH-3. The pattern of expression of the MH-3 gene is intriguing: a) in the adult mouse, this gene is expressed specifically in the testis or ovary b) in the adult testis, this gene is expressed in germ cells at late meiotic stages c) during embryogenesis, the expression of this gene was detected at stages that precede gonadal and sexual differentiation. We will study the MH-3 homeo box gene with respect to its structure and expression and function in meiosis and embryogenesis. 1. The temporal and spatial control of MH-3 gene expression will be analyzed in detail in adult gonads and in mouse embryos by in situ hybridization and indirect immunoflouresence, using specific MH-3 nucleic acid and antibody probes. 2. The MH-3 gene will be isolated and characterized. Cis acting sequences that control the specificity of expression will be studied using hybrid genes in transgenic mice. 3. Loss of MH-3 gene function will be attempted in embryos by microinjecting antisense DNA constructs, antisense RNA or antibodies into fertilized mouse eggs. The embryonic and postnatal development of the manipulated embryos will be studied after transfer into pseudopregnant recipients. 4. To ask whether MH-3 proteins are essential for meiosis, this process will be analyzed in primary oocytes microinjected with antibodies and in spermatogenic cell cultures infected with recombinant retroviruses expressing the MH-3 antisense RNA.