In a recent study in HEPATOLOGY, Werth et al. 1 reported that STAT5 is a transcription factor, which binds to a section of the upstream regulatory region of the glutamine synthetase (GS) gene that, in transfection experiments, enhances reporter-gene expression. The next obvious step to further develop this hypothetical mechanism is to study STAT5 knockout mice and establish if STAT5 signaling in the liver is indeed necessary to mediate GS expression in mammalian liver. Some years ago, we addressed this question in the STAT5 knockout mouse models developed in the laboratory of Dr. Ihle. 2 At the time, we studied GS mRNA expression in STAT5a ϩ/ϩ /b ϩ/ϩ , STAT5a Ϫ/Ϫ /b ϩ/ϩ , STAT5a ϩ/ϩ /b Ϫ/Ϫ , and STAT5a Ϫ/Ϫ /b Ϫ/Ϫ livers with a quantitative in situ hybridization technique. 3 The expression pattern and quantification of GS mRNA in these livers is shown in Fig. 1. Mice lacking the STAT5a protein tended to show slightly higher GS mRNA levels (integrated OD ϭ 0.921 and 0.875 for STAT5a Ϫ/Ϫ /b ϩ/ϩ and STAT5a Ϫ/Ϫ / b Ϫ/Ϫ livers, respectively) compared to mice expressing the STAT5a protein (OD ϭ 0.652 and 0.692 for STAT5a ϩ/ϩ /b ϩ/ϩ and STAT5a ϩ/ϩ /b Ϫ/Ϫ livers, respectively). Figure 1 unequivocally shows that deficiency of either STAT5a or STAT5b, or both, does not diminish the expression of GS in mouse liver. Our findings, of course, do not question the identity of the protein isolated and characterized by Werth et al., but complement their data by demonstrating redundancy at the STAT5 site of the signaling cascade that regulates expression of the GS gene.