Give one major function of RER.

Explanation

**Step-by-Step Solution:**

1. **Identify RER**: The Rough Endoplasmic Reticulum (RER) is a type of endoplasmic reticulum that has ribosomes attached to its surface, giving it a "rough" appearance.

2. **Understand the Structure**: RER is a network of tubular structures that are scattered throughout the cytoplasm and is continuous with the outer membrane of the nucleus.

3. **Recognize the Role of Ribosomes**: Ribosomes are the cellular structures responsible for protein synthesis. Their presence on the RER is crucial for its function.

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