disclosed are a dna sequence encoding a mutant l3t4 protein which , when expressed on the surface of a cell , is capable of facilitating infection of the cell by human immunodeficiency virus ; the mutant protein includes at least one amino acid residue substitution or deletion in a segment corresponding to the gp120 binding epitope of a native l3t4 protein so as to increase homology between that segment and its counterpart in a cd4 protein ; a murine cell line or strain transfected with such a dna sequence ; and a transgenic rodent susceptible to infection by human immunodeficiency virus .