Complex sentence: ERCC1 knockout mice are runted at birth and die before weaning due to liver failure, an unusual phenotype also seen in XPF knockout mice, but not mice deficient in any other nucleotide excision repair protein.
Split into simpler sentences: ERCC1 knockout mice are runted at birth and die from progressive hepatic insufficiency. Liver failure also occurs in XPF knockout mice, but not mice deficient in any other nucleotide excision repair protein.