RNAi gene is not a naturally occurring gene.

Explanation:

Naturally occurring genes:
1. cry-gene: The cry-gene is a naturally occurring gene that is found in certain strains of Bacillus thuringiensis (Bt) bacteria. It codes for a protein called Cry toxin, which has insecticidal properties. This gene is commonly used in genetically modified crops to provide resistance against insect pests.

2. Bt: Bt stands for Bacillus thuringiensis, which is a naturally occurring soil bacterium. It produces various insecticidal proteins, known as Cry toxins, that are toxic to certain insect pests. The Bt gene is obtained from this bacterium and used in genetically modified crops to provide insect resistance.

3. Cellular defence gene: Cellular defence genes are naturally occurring genes that play a role in the immune response of organisms. These genes encode proteins involved in recognizing and responding to pathogens, such as viruses or bacteria. Examples of cellular defence genes include those encoding antimicrobial peptides, cytokines, and immune cell receptors.

RNAi gene:
RNAi (RNA interference) is a process by which gene expression is regulated or silenced through the action of small RNA molecules. RNAi does not refer to a specific gene, but rather a mechanism by which gene expression is controlled. Small RNA molecules, such as microRNAs or small interfering RNAs (siRNAs), can bind to specific mRNA molecules and prevent their translation into proteins. This mechanism is found in many organisms, including plants, animals, and fungi, and is involved in various biological processes. While RNAi is a naturally occurring mechanism, it is not a specific gene in itself.

In summary, the RNAi gene is not a naturally occurring gene. It refers to a mechanism by which gene expression is regulated through the action of small RNA molecules.