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. (Microbial infection) The NELF complex is involved in HIV-1 latency possibly involving recruitment of PCF11 to paused RNA polymerase II. The NELF complex is composed of NELFA, NELFB, NELFCD (isoform NELF-C or isoform NELF-D) and NELFE; NELFA and NELFCD form a stable subcomplex that binds to the N-terminus of NELFB (PubMed:11387440). In vitro, the NELFA:NELFCD subcomplex binds to ssDNA and ssRNA in a sequence- and structure-dependent manner (PubMed:27282391). Interacts with the RNA polymerase II complex when it is not phosphorylated by P-TEFb (PubMed:10199401). Ubiquitous. Expressed in heart, brain, placenta, liver, skeletal muscle, kidney and pancreas. Expressed at lower level in adult lung. Expressed in fetal brain, lung, liver and kidney. The HDAg-like domain is essential for transcriptional repression, and mediates the interaction with the RNA polymerase II complex. Belongs to the NELF-A family. PubMed:12612062 has shown that it is not involved in Wolf-Hirschhorn syndrome. Truncated N-terminus. Extended N-terminus. Extended N-terminus.