NFKB1 or NFKB2 bind to REL, RELA, or RELB to form the NFKB complex. The NFKB1/RELA heterodimer is the most abundant form of NFKB. The NFKB complex is inactivated by I-kappa-B proteins that keep it trapped in the cytoplasm. Phosphorylation of serine residues on the I-kappa B proteins mark them for ubiquitination and destruction, and NFKB translocates to the nucleus where it binds DNA. Transforming protein Tax inhibits p53 transcriptional activity through the NFKB signaling pathway, specifically via the p65 (RelA) subunit. The inhibition of p53 activity is dependent upon phosphorylation of p65 (RelA) at ser536 by the upstream kinase IKKB.