Which of the following is not naturally occuring gene

Options

A. cry-gene
B. Bt-gene
C. RNAi, gene
D. cellular defense gene

Answer

To determine which of the following is not a naturally occurring gene, we will analyze each option provided:

1. **Cryo gene**: This gene is derived from the bacterium *Bacillus thuringiensis*, which produces proteins known as cryoproteins. These proteins are naturally occurring and are used in biotechnology, particularly in genetically modified crops for pest resistance.

2. **BT gene**: The BT gene is essentially another name for the cryo gene, as it also originates from *Bacillus thuringiensis*. Therefore, it is also naturally occurring.

3. **RNAi (RNA interference)**: RNAi is a mechanism that involves the introduction of double-stranded RNA to silence specific genes. This process is often artificially induced in organisms to study gene function or to protect plants from pests. Since RNAi is artificially induced and not a naturally occurring gene, it does not fit the criteria of being naturally occurring.

4. **I gene**: The I gene is associated with cellular defense mechanisms in various organisms. It is involved in the immune response and is naturally occurring.

5. **Cellular defense gene**: This refers to genes that are part of the immune response in organisms. For example, restriction endonucleases are naturally occurring enzymes in bacteria that protect against viral infections. Thus, this gene is also naturally occurring.

After analyzing all the options, we conclude that the **RNAi gene** is the only one that is not naturally occurring.

### Final Answer:
The gene that is not naturally occurring is **RNAi (RNA interference) gene**.