A genome-wide CRISPR screen finds CIP2A as a new synthetic lethal target for BRCA1- and BRCA2-deficient cells. Unlike PARP inhibition that increases replication-induced DNA double-strand breaks and radial chromosomes, depleting CIP2A or disrupting its interaction with TOPBP1 increases micronuclei and chromosomal missegregation, revealing a mitotic target for BRCA-mutated tumors.