This research proposal is based on the hypothesis that the T/t allele mutants on the seventeenth mouse chromosome represent mutant forms of genes determining the structure of cell surface antigens required for cellular interaction and triggering in early embryonic development. To test this hypothesis, the H-2, Ir, and MLC type of chromosomes bearing representatives of each of the six t allele complementation groups will be determined. One representative of each of the t alleles will be established as a balanced lethal line (T/t) on the 129/TerSv and C3H/DiSn backgrounds. These congenic strains will be used for the production of tetraparental mice (by embryo fusion) to determine whether normal embryonic tissues are capable of "rescuing" the t allele defect, to determine whether a given t allele can affect later stages of development than the already described developmental stage, and to produce tetraparental animals whose germinal ridge and testicular tissues are of the T/T or tn/tn genotype. The use of balanced lethal lines on the 129/TerSv background should permit detection and isolation of teratomas of the T/t genotype, which will be tested for expression of antigens coded for by the t alleles. Immunization with adult sperm and teratoma tissue will be used to produce antibodies specific for antigens coded for by the T gene and each of the t alleles. These anti-sperm and anti-teratoma antibodies will be tested for reactivity with sperm, teratocarcinoma tissue, embryonic tissue, and adult tissue to determine the precise stage in embryonic development at which the t allele antigens are expressed. Particular effort will be made to develop a new immunofluorescent binding assay for detecting cell surface antigens on sperm and embryonic tissues.