The Sokal Score for Chronic Myeloid Leukaemia was developed by John Sokal and colleagues in 1984 from a collaborative analysis of 813 CML patients treated with busulphan or hydroxyurea (the pre-TKI era). Despite being developed over 40 years ago, the Sokal score retains prognostic value in the TKI era and is recommended by the European LeukemiaNet (ELN) 2020 guidelines for initial risk stratification of newly diagnosed CML. Patients with high Sokal scores achieve lower rates of deep molecular response (MR4/MR4.5) with imatinib and are more likely to require 2nd-generation TKIs. The Sokal formula uses age, spleen size, platelet count, and peripheral blood blast percentage.