FOS and Miyoshi myopathy: Additionally, CEBPB exerts an inhibitory activity toward the AP1 complex, at both the expression and transcriptional levels (Gagliardi et al., 2003), demonstrated also in a human MM cell line (Takeshita et al., 2004), that may be responsible for the low JUN and FOS levels in the Eμ-xbp-1s B cells.