With DNA lesions induced by environmental physical and chemical factors, actively transcribed genes and DNA transcribed strands were preferentially repaired.This preferential DNA repair directly connected with the process of gene transcription.A transcription-repair coupling factor (TRCF) encoded by mdf gene has been identified and isolated in E. coli. TRCF is a DNA binding protein with ATPase activity. In eukaryotes, some DNA repair proteins were found to be involved in transcription. For example, the largest subunit of general transcription factor TFⅡH, p89, is the encoding product of human excision repair gene ERCC-3 .And the excision repair gene RAD3 of yeast encodes the largest subunit of transcription factor b, p85.