The first restriction endonuclease was:

Options

A. EcoRII
B. Hin d-II
C. Hin d-III
D. Ava I

Answer

**Step-by-Step Solution:**

1. **Understanding Restriction Endonucleases**:
- Restriction endonucleases, commonly known as restriction enzymes, are proteins that cut DNA at specific sequences. These sequences are referred to as recognition sites or restriction sites.

2. **Identifying the First Restriction Endonuclease**:
- The question asks for the first restriction endonuclease discovered.

3. **Reviewing the Options**:
- The options provided are:
- A. ECHO-R2
- B. HIN-2
- C. HIN-3
- D. EVA-1

4. **Researching Historical Context**:
- The first restriction enzyme to be discovered and characterized was HIN-2. It was isolated from the bacterium Haemophilus influenzae strain Rd.

5. **Conclusion**:
- Based on the historical context and the information gathered, the correct answer is **B. HIN-2**.

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