The present invention relates to a non-human transgenic animal, particularly a knock in mouse, whose genome comprises a heterologous, chimeric CTLA4 gene. The chimeric CTLA4 gene comprises exon 2 of the human CTLA4 gene, exon 1 and exon 4 of the non-human animal, and exon 3 of the CTLA4 gene of the non-human animal, or preferably, exon 3 of the human CTLA4 gene. The invention also relates to methods by which the transgenic mice are used to screen for monoclonal antibodies or other molecules that enhance immunity to tumors and infectious agents by interacting with the human CTLA4 receptor. The transgenic mice of the present invention are also useful for screening for monoclonal antibodies or other molecules that inhibit autoimmunity and transplant rejection.