Thymocytes' developmental stages and T-ALL subtypes. The ETP-ALL subtype is driven by aberrant MEF2C or HOXA gene expression, presents frequent mutations in the IL7 signaling cascade, and shows higher BCL2 protein expression. Similarly to hematopoietic progenitors, ETP-ALL blasts express stem cell markers such as CD34. The TLX subgroup, driven by either TLX3- or HOXA-activating events, often presents NOTCH1 mutations and, in some cases, expression of the γ/δ T-cell receptor (TCR), in analogy to the precortical γ/δ T-cell progenitors (DN2 stage). The TLX1/NKX2.1 subgroup is driven by either NKX2.1 or TLX1 aberrations. TLX-rearranged cases can present the oncogenic NUP214–ABL1 fusion. The TAL/LMO subgroup, driven by the expression of the oncogenes TAL1 and LMO2, includes the most mature T-ALL cases. As for late cortical (SP) T-cell progenitors, TAL/LMO blasts express mature T-cell surface markers such as CD4, CD8, CD3, and α/β TCR and often present PTEN mutations.