Essential component of the NELF complex, a complex that negatively regulates the elongation of transcription by RNA polymerase II (Pol II) (PubMed:25773599). 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). May be able to induce chromatin unfolding (By similarity). 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 (PubMed:19340312). 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 (PubMed:25773599). 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 (PubMed:24656816). The NELF complex is composed of NELFA, NELFB, NELFCD and NELFE; the N-terminus of NELFB binds to the NELFA:NELFCD subcomplex (By similarity). Binds RNA which may help to stabilize the NELF complex on nucleic acid (By similarity) Interacts with the first BRCT repeat of BRCA1 (By similarity). Interacts with KIAA1191 (By similarity). Isoform 1 and isoform 2 interact with NELFA, NELFCD and NELFE (PubMed:26010750). Isoform 1 is expressed in the kidney, liver, adipose and lung (PubMed:26010750). Isoform 2 is widely expressed (PubMed:26010750). Mice exhibit early embryonic lethality (PubMed:25773599, PubMed:19340312). Knockdown in embryonic stem cells (ESCs) leads to proliferation defects, increased differentiation, increased expression of development-associated genes and the dysregulation of genes involved in signaling and metabolic pathways (PubMed:25773599, PubMed:19340312). Mice exhibit cardiomyopathy, impaired response to cardiac stress and the reduced expression of metabolism-related genes in cardiomyocytes (PubMed:24656816). Produced by alternative initiation at a CTG start codon. Belongs to the NELF-B family. Truncated N-terminus. Truncated N-terminus.