Hydroxymethylbilane is a linear tetrapyrrole compound that plays a crucial role in the biosynthesis of heme, an essential component of hemoglobin in red blood cells. It is formed from the condensation of porphobilinogen molecules during the heme synthesis pathway. This compound is an intermediate in the production of biliverdin and ultimately bilirubin, which are important for the breakdown of hemoglobin. Hydroxymethylbilane is typically found in the liver and is involved in various metabolic processes related to the body's response to oxygen and the recycling of iron.