Characteristics of Clinical Response in Patients With Chronic Myeloid Leukemia With an ABL1 In‐Frame Exon 4 Deletion: A Multicenter Retrospective Study

Introduction In‐frame Exon 4 deletion (p.L184_K274del) is a rare ABL1 splicing variant in chronic myeloid leukemia (CML), the clinical significance of which remains unclear. Methods We retrospectively analyzed seven chronic‐phase CML patients with this variant, identified by Sanger sequencing during routine monitoring of atypically slow molecular responses. Results All patients achieved hematologic responses; molecular responses, although attained, were often delayed. No patient progressed to advanced phase disease during a median follow‐up of 39.0 months. Conclusion Detection of this kinase‐inactive variant should not, in itself, be regarded as a definitive resistance mechanism. Larger collaborative studies using quantitative methodologies are needed. Trial Registration The authors have confirmed clinical trial registration is not needed for this submission.