- The S box of major histocompatibility complex class II promoters is a key determinant for recruitment of the transcriptional co-activator CIITA. Muhlethaler-Mottet, A., Krawczyk, M., Masternak, K., Spilianakis, C., Kretsovali, A., Papamatheakis, J., Reith, W. J. Biol. Chem. (2004)