A novel calcitonin-encoding mRNA is produced by alternative processing of calcitonin/calcitonin gene-related peptide-I pre-mRNA.

The calcitonin/calcitonin gene-related peptide-I (CALC-I) gene was one of the first examples of a cellular gene exhibiting alternative, tissue-specific processing of its primary RNA transcript. Calcitonin (CT) mRNA is the predominant product in thyroid C cells, whereas calcitonin gene-related peptide-I mRNA is the main product in neurons of the central and peripheral nervous systems. In this study, we report the existence of a novel CT-like (CT*) mRNA derived from the human CALC-I gene. CT* mRNA is the result of a novel splicing reaction which was initially detected in vitro. Translation of CT* mRNA will result in a prohormone (proCT*) which only differs from proCalcitonin (proCT) in its carboxyl terminus. Proteolytic processing of proCT* predicts, beside the amino-terminal cleavage peptide and CT, the existence of a novel carboxyl-terminal protein of 21 amino acids.