Cytochrome : Chromoprotein :: Casein : ________

Explanation

To solve the analogy "Cytochrome : Chromoprotein :: Casein : ________", we need to identify the relationship between the first pair of terms and apply that to the second pair.

### Step-by-Step Solution:

1. **Identify the first term and its classification**:
- Cytochrome is a type of protein known as a chromoprotein.
- A chromoprotein is a conjugated protein that contains a pigmented prosthetic group.

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