In which cancer type, no tumour is formed.

Explanation

**Step-by-Step Solution:**

1. **Understanding Tumors**: Begin by defining what a tumor is. Tumors are masses of tissue that arise from the uncontrolled growth of cells. Normal cells grow and divide in a regulated manner, but when they lose this regulation, they can form tumors.

2. **Types of Tumors**: Differentiate between benign and malignant tumors. Benign tumors are localized and do not spread to other parts of the body, while malignant tumors (cancers) are characterized by their ability to invade surrounding tissues and metastasize (spread) to other parts of the body.

3. **Identifying Cancer Without Tumors**: The question specifically asks for a type of cancer where no tumor is formed. In most cancers, tumors are present, but certain types of cancer do not form solid masses.

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