Most fluorophores are organic small molecules of 20 - 100 atoms (200 - 1000 Dalton - the molecular weight may be higher depending on grafted modifications, and conjugated molecules), but there are also much larger natural fluorophores that are proteins: Green fluorescent protein (GFP) is 27 kDa and several phycobiliproteins (PE, APC...) are ≈240kDa.