Our case represents a striking example of myelomastocytic transformation in the blast phase of chronic myeloid leukemia (CML), characterized by key diagnostic findings: elevated immature myelomastocytic cells (50% in peripheral blood, 22% in bone marrow) with metachromatic granules, along with strong CD117 positivity and tryptase expression. The gene discussed is KIT; the disease is chronic myelogenous leukemia, BCR-ABL1 positive.