Scientists have identified a group of surface markers on cells linked to an aggressive type of breast cancer called estrogen receptor-negative cancer. In this preliminary study, estrogen-negative breast cancer developed when three markers, CD44+, CD49fhi, and CD133hi were present simultaneously on the surface of human cells taken from breast cancer patients and transplanted into a mouse;

this is called a xenograft model. The scientists named these human cells with tumor-forming ability in mice, xenograft-initiating cells, or XIC. The research, conducted by scientists at the National Cancer Institute (NCI), part of the National Institutes of Health, appeared online May 18, 2010, and in print June 1, 2010, in Cancer Research.