Chemically, bilins are linear arrangements of four pyrrole rings (tetrapyrroles). In human metabolism, bilirubin is a breakdown product of heme. Hydroxymethyl bilane is a major anabolic product, from the biosynthetic reaction of porphobilinogen (PBG) and uroporphyrinogen I synthase (known as porphobilinogen deaminase).
Examples of bilins are found in animals, and phycocyanobilin, the chromophore of the photosynthetic pigment phycocyanin in algae and plants. In plants, bilins also serve as the photopigments of the photoreceptor protein phytochrome. An example of an invertebrate bilin is micromatabilin, which is responsible for the green color of the Green Huntsman Spider, Micrommata virescens.