When these Stat5a − / − mice were bred to MMTV - tva mice, infected with RCAS - caErbb2, impregnated 4 days post infection, and analyzed at I10,
the resultant early lesions exhibited significantly lower levels of pSTAT5 than did the lesions in Stat5a + / + mice (Figure 5A) confirming the predominance of STAT5a in the mammary gland.