Expression of either STAT5A/WKR or STAT5A/▵53C in 32D cells inhibits IL-3–dependent proliferation. 32D cells expressing the neomycin resistance gene alone (32D/Neo), STAT5A/WKR (32D/WKR), or STAT5A/▵53C (32D/▵53C) were grown in 10% WEHI-3B–conditioned medium without G418 and viable cell number was determined daily. Data points represent the average of at least two independent experiments performed in duplicate and error bars represent the calculated standard error. The day-2 cell number for 32D/Neo cells was significantly higher than that of either 32D/WKR or 32D/▵53C (P < .01,t-test).