For years, it has been unclear how genetic defects in the SUMO pathway may cause enormous chromosomal translocations, and this question has captured our attention. Our latest work provided the first clue, suggesting that SUMO modification of MCM controls its origin-bound levels. This finding is both surprising and exciting, and it provided an elegant explanation for the cause of chromosomal translocations - reduced MCM loading at DNA replication origins may cause incomplete DNA replication, leading to broken and un-replicated DNA, the precursor of chromosomal translocations.