Researchers have found a protein that helps breast cancer cells spread and found it in the urine of women with aggressive breast cancer — offering a potentially painless way to warn a patient.

The protein and the gene that controls it are called lipocalin 2, or Lcn2. The team at Children's Hospital Boston showed not only that it helps the tumors spread through the body, but can be detected in a simple urine test.

"Lcn2 is among the genes most highly associated with estrogen receptor (ER)-negative breast tumors," Marsha Moses, Jiang Yang and colleagues at Children's wrote in Monday's issue of the Proceedings of the National Academy of Sciences.

Estrogen-negative tumors are more difficult to treat because the widely used drug tamoxifen, and newer drugs called aromatase inhibitors, have little effect on them.

Lcn2 was known to leak from breast tumors into the breast ducts. "We considered the possibility that Lcn2 might be detected in body fluids and might be associated with disease status," Moses and colleagues wrote.

"We analyzed Lcn2 levels in urine samples from healthy women and women with metastatic breast cancer," they added.

Women whose cancer had been known to spread, or metastasize, had higher levels of the compound in their urine.

"Our study identifies a novel, additional player in the complex development of invasive breast cancer," Moses said in a statement. Drugs that attack the protein may also help treat breast cancer, she said.