IL7R (Interleukin 7 receptor alpha) is required for normal lymphocyte development. Loss of function mutations are seen in severe combined immunodeficiency. More recently, heterozygous, somatic, activating mutations have been described in pediatric B-cell and T-cell acute lymphoblastic leukemia. These mutations are most frequently in-frame insertions and deletions in the transmembrane domain. In general, these mutations lead to the addition of a cysteine residue in the juxtamembrane domain, a change that is essential for the resultant constiutive activation of the receptor and JAK/STAT and mTOR pathways. Recently, non-cysteine mutations have been described in the transmembrane domain of IL7R, some of which are activating. IL7R mutations have been described in up to 6% of childhood B-ALL and are typically associated with aberrant CRLF2 expresssion and in up to 10% of childhood T-ALL/adult T-ALL and may co-exist with NOTCH1 mutations. These mutations are rare in adult AML(1%). The prognostic significance of these mutations remains to be elucidated. These mutations may have implications for targetted therapy. In addition to in frame exon 6 in/dels, activating mutations in exon 5 have been described in IL7R which are not detected by this assay.