For protein targets, probes are typically _antibodies_ or _aptamers_ that specifically bind to the target protein. The design process includes:
1. Epitope Selection: Identifying a specific epitope on the target protein that the probe will bind to. 2. Affinity Optimization: Ensuring that the probe has a high affinity for the target epitope. 3. Cross-Reactivity Testing: Verifying that the probe does not cross-react with other proteins. 4. Labeling: Attaching a detectable label to the probe.
