Fill in the blanks:
______________ is an exception to cell theory.

Explanation

**Step-by-Step Solution:**

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

2. **Identifying Exceptions**: While the cell theory applies to most living organisms, there are exceptions. One notable exception is an entity that does not fit the criteria of being classified as a living organism in the traditional sense.

3. **Recognizing the Exception**: The entity that is often cited as an exception to cell theory is the virus. Viruses do not have a cellular structure and cannot carry out metabolic processes on their own. They require a host cell to replicate and function.

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