ERBB2 and breast cancer: In order to combine the favorable particle biodistribution profiles with the high target specificity and affinity of Abs, Ab fragments are a promising alternative.[62] To this end, anti-HER2 single-chain fragments were recently conjugated to dual-modality (PET-optical) C′ dots for imaging HER2 overexpressing breast cancer,[63] leading to a sub-8-nm target-or-clear tracer with high tumor-targeting efficiency, target-to-background ratios, and bulk renal clearance.