The dual-family peptidylprolyl cis-trans isomerases (immunophilins) represent naturally occurring chimera of classical FK506-binding protein (FKBP) and cyclophilin (CYN), connected by a flexible linker, and are found exclusively in monocellular organisms. The modular builds of these molecules represent two distinct types: CYN-(linker)-FKBP and FKBP-3TPR-CYN. Abbreviated respectively as CFBP and FCBP, the two classes also exhibit distinct organism preference, the CFBP being found in prokaryotes, and FCBP, in eukaryotes. This review summarizes the mystery of these unique class of prolyl isomerases, focusing on their host organisms, potential physiological role, and likely routes of evolution.