Transgenic mice have been generated that carry a CD45 minigene under control of the human leukocyte function-associated antigen (LFA-1, CD11a) promoter. CD45-null mice carrying the transgene exhibit the lymphocyte lineage-specific isoform expression patterns of wild-type mice. Furthermore, these mice have normal thymocyte development and peripheral T-cell numbers. The proliferative ability of T cells in response to mitogens and antigen also is regained, as is B-cell responsiveness to anti-IgM. The antibody response to antigen is also restored and is similar to that of normal mice. Therefore, introduction of a functional CD45 minigene is sufficient to overcome the principal severe combined immunodeficiency (SCID)-associated defects and represents a potential route to a gene therapy for human CD45-deficent SCID.