Essential component of the NELF complex, a complex that negatively regulates the elongation of transcription by RNA polymerase II. The NELF complex, which acts via an association with the DSIF complex and causes transcriptional pausing, is counteracted by the P-TEFb kinase complex. The NELF complex is composed of 4 subunits: WHSC2/NELF-A, COBRA1/NELF-B, TH1L (isoform NELF-C or isoform NELF-D) and RDBP/NELF-E. Interacts with ARAF1. Widely expressed. Expressed in heart, brain, lung, placenta, liver, skeletal and cardiac muscle, adrenal, thyroid, kidney and pancreas. Belongs to the NELF-D family. 3 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.