Transgenic mice having a phenotype characterized by the substantial depletion of a mature lymphocytic cell type otherwise naturally occurring in the species from which the transgenic mouse is derived. The phenotype is conferred in the transgenic mouse by a transgene contained in at least the precursor stem cell of the lymphocytic cell type which is depleted. The transgene comprises a DNA sequence encoding a lymphatic polypeptide variant which inhibits maturation of the lymphocytic cell type.