As the RUNX1 gene is often found in the highest region of chromosome 21 gain, fluorescence in situ hybridization (FISH) probes directed to the RUNX1 gene as part of the ETV6::RUNX1 fusion probe set is an efficient and cost‐effective method to identify iAMP21‐ALL within clinical cytogenetics laboratories. The gene discussed is RUNX1; the disease is acute lymphoblastic leukemia.