Essential component of the NELF complex, a complex that negatively regulates the elongation of transcription by RNA polymerase II (By similarity). The NELF complex, which acts via an association with the DSIF complex and causes transcriptional pausing, is counteracted by the P-TEFb kinase complex (By similarity). The NELF complex is composed of NELFA, NELFB, NELFCD and NELFE; NELFA and NELFCD form a stable subcomplex that binds to the N-terminus of NELFB (By similarity). In vitro, the NELFA:NELFCD subcomplex binds to ssDNA and ssRNA in a sequence- and structure-dependent manner (By similarity). Interacts with the RNA polymerase II complex when it is not phosphorylated by P-TEFb (By similarity). Interacts with NELFB (PubMed:26010750). Ubiquitous. Expressed in brain, heart, spleen, lung, liver, muscle, kidney and testis. Already expressed in 7 dpc embryos. 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.