The EMSY gene encodes a protein that interacts with Brca2 and is amplified in some sporadic cases of human breast cancer. To examine whether overexpression of EMSY would mimic the chromosome instability phenotype that is associated with the loss of Brca2 function, we constructed a lentiviral vector (Lenti-EMSY/GFP) that encodes a truncated form of the Emsy protein, including its Brca2-interacting domain, and green fluorescent protein (GFP) and used it to transduce human telomerase-immortalized human breast epithelial (184-hTert) cells, which have a nearly normal karyotype. At passage 5 after transduction, 39 (26%) of 150 EMSY/GFP-transduced metaphase cells contained at least one structural chromosomal abnormality compared with 19 (13%) of 150 GFP-transduced metaphase cells (P = .003, chi-square test); at passage 10, the corresponding frequencies were 42% and 15%, respectively (P < .001). Mitomycin C also produced a severalfold higher frequency of chromosome breaks in the EMSY/GFP-transduced cells than in the control cells. These results support the hypothesis that EMSY overexpression can play a role in the genesis of human breast cancer.