Mutations have also been reported in the signal transducer and activator of transcription gene, STAT5B, which is part of the signaling cascade of the GH receptor; in addition to growth failure, clinical features of these patients frequently involve immunodeficiency and pulmonary fibrosis (24, 26), although some missense mutations of STAT5B have recently been reported to not cause severe immune and pulmonary problems (27). Here, STAT5B is linked to immunodeficiency disease.