PPIB and connective tissue disorder: Hyperelastosis cutis is a connective tissue disorder caused by the Gly6->Arg mutation in cyclophilin B. The high resolution structures of the recombinant wild-type and mutated horse cyclophilin B provide evidence that the active enzymatic site is undisturbed and suggesting that the disease phenotype is caused by changes in the structure and flexibility of the N-terminal tail, altering interactions with other ER-resident chaperones and foldases.