Pin1, a phosphorylation-dependent peptidyl-prolyl cis/trans isomerase (PPIase), regulates the activity of a number of cell cycle regulators, transcription factors, and microtubule-associated tau. Aberrant expression of Pint is implicated in carcinogenesis and neurodegenerative diseases. Yet, there are discrepancies regarding its biological significance in different organisms. Pin1 was essential in HeLa cells, while Pin1-deficient mice showed no lethal phenotypes. We here identified a novel murine Pin 1 isoforna (mPin 1L) consisting of the WW domain and the PPIase domain. Murine Pin I L shares 92% sequence identity with the wild-type Pin I and shows wide tissue distribution with highest levels in mouse testis. The recombinant mPin I L is enzymatically active, but is approximately three times less efficient than Pin1 in catalyzing the cis/trans isomerization. These data suggest that mPin1L may serve as a surrogate for Pin1. The finding provides insights into phenotypic consequences for Pin1-null mice and may facilitate future biological study and pharmacological development in mice. (c) 2007 Elsevier Inc. All rights reserved.