Define selectable marker.

Explanation

### Step-by-Step Text Solution:

1. **Understanding the Context**: In recombinant DNA technology, scientists often want to introduce a specific gene into a host organism (like bacteria) to study its function or produce a desired product.

2. **Defining Transformation**: Transformation is the process by which a piece of DNA is introduced into a host cell. This is crucial for the success of recombinant DNA experiments.

3. **Role of Selectable Markers**: Selectable markers are specific genes included in the vector (the DNA molecule used to deliver the gene of interest) that allow researchers to identify which cells have successfully taken up the vector.

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