The NF-kB/Rel transcription factors are present in the cytosol in an inactive state complexed with the inhibitory IkB proteins.The regulation of IkBbeta and IkBE is similar to that of IkBalpha. However, the phosphorylation and ubiquitin-mediated degradation of these proteins occurs with much slower kinetics. IKK phosphorylation of IkBbeta occurs at Ser19 and Ser23, while IkBE can be phosphorylated at Ser18 and Ser22 . The human sequence of IkB-beta has also been reported to contain a threonine at position 19 suggesting that phosphorylation could be Thr19/Ser23.