Mouse Ubc9 knockout: many path(way)s to ruin.

In this issue of Developmental Cell, Nacerddine et al. (2005) describe a targeted germ line mutation of the mouse SUMO-specific E2 Ubc9, which abrogates the SUMO conjugation pathway, broadly crippling nuclear function in proliferating cells of the early embryo. This study reveals the wide-ranging roles for this protein modifier in nuclear organization, chromosome segregation, and Ran-driven nucleo-cytoplasmic transport.