Essential component of the NELF complex, a complex that negatively regulates the elongation of transcription by RNA polymerase II (PubMed:12612062). The NELF complex, which acts via an association with the DSIF complex and causes transcriptional pausing, is counteracted by the P-TEFb kinase complex (PubMed:10199401). (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 primarily through NELFCD to the N-terminus of NELFB (PubMed:27282391, PubMed:12612062). Binds RNA which may help to stabilize the NELF complex on nucleic acid (PubMed:27282391). In vitro, the NELFA:NELFCD subcomplex binds to ssDNA and ssRNA in a sequence- and structure-dependent manner (PubMed:27282391). Interacts with ARAF (PubMed:11952167). Interacts with PCF11 (PubMed:23884411). Interacts with KAT8 (By similarity). Widely expressed. Expressed in heart, brain, lung, placenta, liver, skeletal and cardiac muscle, adrenal, thyroid, kidney and pancreas. Produced by alternative initiation at Met-10 of isoform NELF-C. Belongs to the NELF-D family.