Stability of vav-hCD4 transgene expression. hCD4 levels on viable peripheral blood leukocytes from individual HS21/45 mice, of 2 independent lines, at more than 1 year of age (A and B). The proportion of cells with transgene-associated fluorescence above that from a nontransgenic littermate (dotted and shaded histogram) is given. (C) Maintenance of full transgene activity in individual vav-hCD4mice (open symbols, representing 3 different lines) versus the decrease in mice of a typical vav-lacZ line (solid symbols).