STAT5B and immune system disorder: In addition to growth failure, most children with homozygous inactivating STAT5B mutations, have an associated immunodeficiency and pulmonary fibrosis.28 Moreover, a heterozygote condition for a STAT5B mutation has been described.29 Scalco et al. showed that heterozygous STAT5B mutations have a significant negative impact on height.29 However, the height is usually within the normal range, demonstrating a milder effect than that resulting from homozygous mutations.