In humans, IL-7 produced by stromal cells of lymphoid tissues and by hepatocytes is the true "T cell growth development factor" [55], and so its dosage in Guthrie card eluate is a complementary (respect to TRECs test) method for newborn SCID screening: blood IL-7 is increased "by feedback" if low T lymphocytes, and newborn IL-7 levels > 15 pg/mL indicate T- SCID [56]; note that IL-7 levels may be normal in T+ or T++ SCID (e.g., massive engraftment of maternal T lymphocytes, and Omenn syndrome; see before), and therefore TRECs test appears to be the best method for newborn SCID screening. Here, IL7 is linked to Omenn syndrome.