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). May be able to induce chromatin unfolding (PubMed:11739404). Essential for early embryogenesis; plays an important role in maintaining the undifferentiated state of embryonic stem cells (ESCs) by preventing unscheduled expression of developmental genes (By similarity). Plays a key role in establishing the responsiveness of stem cells to developmental cues; facilitates plasticity and cell fate commitment in ESCs by establishing the appropriate expression level of signaling molecules (By similarity). Supports the transcription of genes involved in energy metabolism in cardiomyocytes; facilitates the association of transcription initiation factors with the promoters of the metabolism-related genes (By similarity). (Microbial infection) The NELF complex is involved in HIV-1 latency possibly involving recruitment of PCF11 to paused RNA polymerase II (PubMed:23884411). In vitro, binds weakly to the HIV-1 TAR RNA which is located in the long terminal repeat (LTR) of HIV-1 (PubMed:23884411). The NELF complex is composed of NELFA, NELFB, NELFCD (isoform NELF-C or isoform NELF-D) and NELFE; the N-terminus of NELFB binds to the NELFA:NELFCD subcomplex (PubMed:12612062). Binds RNA which may help to stabilize the NELF complex on nucleic acid (PubMed:27282391). Interacts with the first BRCT repeat of BRCA1 (PubMed:11739404). Interacts with KIAA1191 (PubMed:21153684). Interacts with NELFE (By similarity). Widely expressed. Expressed in heart, brain, lung, placenta, liver, skeletal muscle, kidney and pancreas. Produced by alternative initiation at a CTG start codon. Belongs to the NELF-B family. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus. Truncated N-terminus.