We report the cloning of a mouse cDNA encoding a novel protein which has significant homology with the t-complex protein 1b (TCP1b). In addition, this protein has high sequence identity with tryptic peptides from the bovine P5 subunit of the TCP1-ring complex. We named this novel protein mTRiC-P5 for mouse TCP1-Ring Complex Protein #5. Results indicate that mTRiC-P5 is a new member of the TCP1-TF55 family and is part of the TCP1-ring complex.