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 primarily through NELFCD to the N-terminus of NELFB (By similarity). Binds RNA which may help to stabilize the NELF complex on nucleic acid (By similarity). In vitro, the NELFA:NELFCD subcomplex binds to ssDNA and ssRNA in a sequence- and structure-dependent manner (By similarity). Interacts with ARAF1 (By similarity). Interacts with PCF11 (By similarity). Interacts with NELFB (PubMed:26010750). Interacts with KAT8 (PubMed:17335777). Belongs to the NELF-D family.