Mutations in the interleukin-7 receptor (IL7R) or the JAK3 kinase occur frequently in T-cell acute lymphoblastic leukemia (T-ALL) and both are able to drive cellular transformation and the development of T-ALL in mouse models. However, the signal transduction pathways downstream of JAK3 mutations remain poorly characterized. Here, we describe the phosphoproteome downstream of the JAK3(L857Q)/(M511I) activating mutations in transformed Ba/F3 lymphocyte cells and human JAK3 mutated T-ALL samples . Novel peptides shown to be downstream of mutant JAK3 regulating RNA metabolism as well as epigenetic and apoptotic processes were validated using targeted PRM proteomics.