There are several exceptions to the cell theory, .......... is one of them.

Explanation

**Step-by-Step Solution:**

1. **Understanding Cell Theory**: The cell theory was proposed by scientists Schleiden and Schwann. It states that all living organisms are composed of cells, and that the cell is the basic unit of life.

2. **Identifying Exceptions**: While the cell theory is widely accepted, there are exceptions to this theory. These exceptions are cases where the basic principles of the cell theory do not hold true.

3. **Example of an Exception**: One of the most notable exceptions to the cell theory is the virus. Viruses are considered living entities, but they do not possess a cellular structure.

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