A<unk>  is a metalloprotein that serves a variety of important functions, its main being O2 transport. Other functions performed include O2 storage, CO2 transport, and transportation of substances other than<unk>  gases. There are four major classifications of<unk> : hemoglobin, hemocyanin, erythrocruorin–chlorocruorin, and hemerythrin. The heme-containing globin is the most commonly-occurring<unk> , occurring in at least 9 different phyla of animals.
Respiratory pigment
Melanin
Hemosiderin
Vapours 
Hemichrome
Protein corona
Silver nanoparticle
Mustard gas