Using the current definition of iAMP21‐ALL (greater than or equal to three extra copies of RUNX1 on a single abnormal chromosome 21 and greater than or equal to five copies of the RUNX1 gene region per cell), 188 (91%) cases met this definition and were classified as “typical” iAMP21‐ALL, while 19 (9%) cases failed to meet this definition and were classified as “unusual” iAMP21‐ALL (Figure 1A, Figure S1). This evidence concerns the gene RUNX1 and acute lymphoblastic leukemia.