Tonegawa compared the DNA of B cells ( a type of white blood cell ) in embryonic and adult mice . He found that genes in the mature B cells of the adult mice are moved around , recombined , and deleted to make the many versions of the variable region of antibodies . Comparing the DNA of B cells ( a type of white blood cell ) in embryonic and adult mice , he observed that genes in the mature B cells of the adult mice are moved around , recombined , and deleted to form the diversity of the variable region of antibodies .