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PubMed This is a summary of 17 peer-reviewed journal articles Updated
Hematology · Chronic Myeloid Leukemia

The Science of CML: Biology, Phases, and Subtypes

At a Glance

CML is usually driven by the BCR::ABL1 fusion created by the Philadelphia chromosome. Doctors use blast and basophil levels, platelet counts, and additional chromosome changes to classify the disease phase, while the transcript type helps guide molecular monitoring.

Chronic Myeloid Leukemia (CML) is a unique form of cancer because its “engine” is almost always a single, well-defined genetic mistake. Understanding this biology helps explain why your treatment is so targeted and how your medical team determines the “phase” of your disease.

The Genetic “Engine”: BCR::ABL1

CML begins with a specific event called a translocation. This occurs when two chromosomes, numbers 9 and 22, break and swap pieces [1]. The result is a shortened chromosome 22, famously known as the Philadelphia chromosome [2].

This swap fuses two genes together: BCR (from chromosome 22) and ABL1 (from chromosome 9), creating the BCR::ABL1 fusion gene [1]. This new gene produces a protein called tyrosine kinase that is “constitutively active”—meaning it is stuck in the “on” position [3].

This stuck switch floods your body with signals that tell your bone marrow to:

  • Overproduce: Create massive amounts of white blood cells (myeloid cells) [4].
  • Survive: Ignore the signals that usually tell old or damaged cells to die (apoptosis) [4].
  • Crowd Out: Occupy the space in your marrow needed for healthy red blood cells and platelets [5].

The Phases of CML

Doctors categorize CML into phases based on how many immature white blood cells, called blasts, are in your blood or bone marrow, as well as platelet levels, basophils, and clonal evolution. Note that classification systems differ: the recent WHO 5th edition folded the accelerated phase into the chronic phase with high-risk features, while the ICC and ELN still recognize an accelerated phase with different blast thresholds [6].

Phase Description Key Indicators
Chronic Phase (CP) The most common phase (approx. 90-95% of diagnoses). The disease is slow-moving, and cells are relatively mature [7]. Fewer than 10% blasts; fewer than 20% basophils (a type of white cell) [7].
Accelerated Phase (AP) A middle stage recognized by ICC/ELN where the disease begins to move faster, often with persistent low platelets or new chromosomal changes [6]. 10% to 19% blasts; or very high basophil counts (over 20%) [7][8].
Blast Phase (BP) Also called Blast Crisis. The disease behaves like an acute, fast-moving leukemia. Cells no longer mature and quickly overwhelm the body [1][9]. 20% or more blasts (WHO/ICC) or 30% or more (ELN); or leukemia cells found outside the bone marrow [6][7].

Genetic Subtypes: Transcripts

While almost all CML involves BCR::ABL1, the exact “break point” where the genes fused can vary. These different versions are called transcripts.

  • Common Transcripts (e13a2 and e14a2): About 95% of patients have one of these two types [10]. Research suggests that people with the e14a2 transcript may reach deep molecular responses slightly faster on certain medications, but long-term survival is generally excellent for both [10][11].
  • Rare Transcripts (e1a2, e6a2, e19a2): These occur in a small number of patients. Some, like e1a2 or e6a2, may be associated with a more aggressive disease course or a different response to standard drugs [12][13]. If you have a rare transcript, your doctor may use specialized “patient-specific” lab tests to track your progress, as standard tests are designed for the common types [14].

Distinguishing CML from Similar Conditions

Because CML involves high blood counts, it can sometimes be confused with other myeloproliferative neoplasms (MPNs)—a group of diseases where the bone marrow makes too many cells.

  • Polycythemia Vera (PV) primarily involves too many red blood cells [5].
  • Essential Thrombocythemia (ET) involves too many platelets [15].
  • Primary Myelofibrosis (PMF) involves scarring (fibrosis) in the bone marrow [15].

The presence of the Philadelphia chromosome or the BCR::ABL1 gene is a defining finding that strongly supports the diagnosis of CML. However, it is not a standalone “gold standard” because BCR::ABL1 can also be found in Philadelphia-positive acute lymphoblastic leukemia. Your diagnosis requires your doctor to interpret the genetic test alongside your blood counts and clinical context [16]. In very rare cases (about 0.4%), a patient may have both CML and another MPN driver mutation like JAK2, which requires a specialized approach to treatment [17].

Common questions in this guide

What is the Philadelphia chromosome in CML?
The Philadelphia chromosome is a shortened chromosome 22 created when pieces of chromosomes 9 and 22 swap places. This change joins the BCR and ABL1 genes into the BCR::ABL1 fusion gene. The fusion produces a continuously active protein signal that drives excess growth of myeloid blood cells.
How do doctors determine which phase of CML I have?
Doctors examine blood and bone marrow for the percentage of immature white blood cells called blasts, along with basophil levels, platelet counts, and additional chromosome changes. Chronic phase generally has fewer than 10% blasts, while accelerated phase under ICC and ELN is 10% to 19%; blast phase is at least 20% under WHO and ICC or 30% under ELN. The classification system used on your report matters because thresholds differ.
What do e13a2 and e14a2 mean on a CML test?
They are common BCR::ABL1 transcript types, named for the points where the BCR and ABL1 genes joined. About 95% of people with CML have one of these two transcripts. The e14a2 type may reach a deep molecular response slightly faster with some medicines, but long-term outcomes are generally excellent for both.
How are rare CML transcripts monitored?
Rare transcripts such as e1a2, e6a2, and e19a2 may not be measured reliably by standard molecular tests designed for common transcripts. Your care team may use a specialized, patient-specific laboratory test to follow the amount of BCR::ABL1 over time. The test approach should be discussed with your hematologist.
Does a positive BCR::ABL1 test confirm CML?
A BCR::ABL1 result strongly supports CML, but it is not interpreted by itself. The same fusion can occur in Philadelphia chromosome-positive acute lymphoblastic leukemia, so doctors combine the genetic result with blood counts, bone marrow findings, and the clinical picture. This broader evaluation helps distinguish CML from similar disorders.
How is CML different from other myeloproliferative neoplasms?
CML is defined by the BCR::ABL1 fusion and the Philadelphia chromosome in the appropriate clinical setting. Polycythemia vera mainly causes excess red blood cells, essential thrombocythemia mainly causes excess platelets, and primary myelofibrosis involves scarring in the bone marrow. Blood, marrow, and genetic results help doctors tell these conditions apart.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Which classification system did my pathology report use to determine my disease phase (WHO or ICC)?
  2. 2.What was the exact percentage of 'blasts' and 'basophils' found in my blood and bone marrow?
  3. 3.Does my BCR::ABL1 transcript (e13a2, e14a2, or another type) change how you will monitor my response to treatment?
  4. 4.Did my bone marrow biopsy show any 'additional chromosomal abnormalities' beyond the Philadelphia chromosome?
  5. 5.If my transcript is a rare type (like e1a2 or e6a2), will we need a specialized lab to track my molecular response?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

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This page explains the biology, phases, and genetic subtypes of CML for informational purposes only and does not constitute medical advice. Your hematologist and pathologist should interpret your blood, bone marrow, and molecular results and advise you about treatment.

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