The search for sensitive and specific markers of systemic infection has shown that procalcitonin levels are increased in sepsis, and, consequently, this plasma protein has come into the focus of clinical research. Human procalcitonin is encoded by the Calc-l gene, which gives rise to two alternatively spliced transcripts. Despite systemic investigation of the Calc-l gene and mechanisms of the tissue-specific regulation and alternative splicing, little is known about the biology of procalcitonin and the cells which express this protein during inflammation. Here we focus on the molecular and biochemical properties of the molecule and summarize the known biological functions of procalcitonin. We report on the structure of the Calc-l gene, the amino acid conservation of procalcitonin in different species, and the consensus sequences of the protein with regard to sites relevant for posttranslational modification, spatial distribution, and homologies to other cytokines. We discuss aspects of intracellular location of procalcitonin and demonstrate that it has the characteristics of a secreted protein.