The folding of both wild-type and mutant forms of the cystic-fibrosis transmembrane-conductance regulator (CFTR), a plasma-membrane chloride-ion channel, is inefficient 1, 2, 3, 4. Most nascent CFTR is retained in the endoplasmic reticulum and degraded by the ubiquitin proteasome pathway 5, 6, 7. Aberrant folding and defective trafficking of CFTRΔF508 is the principal cause of cystic fibrosis 3, 8, 9, but how the endoplasmic-reticulum quality-control system targets CFTR for degradation remains unknown. CHIP is a cytosolic U-box protein that interacts with Hsc70 through a set of tetratricorepeat motifs 10. The U-box represents a modified form of the ring-finger motif that is found in ubiquitin ligases 11 and that defines the E4 family of polyubiquitination factors 12, 13. Here we show that CHIP functions with Hsc70 to sense the folded state of CFTR and targets aberrant forms for proteasomal degradation by promoting their ubiquitination. The U-box appeared essential for this process because overexpresion of CHIPΔU-box inhibited the action of endogenous CHIP and blocked CFTR ubiquitination and degradation. CHIP is a co-chaperone that converts Hsc70 from a protein-folding machine into a degradation factor that functions in endoplasmic-reticulum quality control.