- Ring1A is a transcriptional repressor that interacts with the Polycomb-M33 protein and is expressed at rhombomere boundaries in the mouse hindbrain. Schoorlemmer, J., Marcos-Gutiérrez, C., Were, F., Martínez, R., García, E., Satijn, D.P., Otte, A.P., Vidal, M. EMBO J. (1997)