New research has shown that scientists may have been looking in the wrong place in their search for the most aggressive forms of breast cancer.
According to researchers, tumours arise in two types of glandular tissue in the breast - known as the outer "basal" cells and inner "luminal" cells.
It was previously thought that aggressive and deadly forms of breast cancer were formed in the basal stem cells.
And scientists believed milder forms of the disease arose in the luminal cells.
The findings, reported in the journal Cell Stem Cell, could lead to new ways in which cancer is treated, and to new prevention strategies being adopted.
The research shows that the majority of breast tumours with defective BRCA1 genes have basal-like characteristics.
This is one of the most deadly forms of cancer.
But scientists who conducted studies on mice to confirm the origin of BRCA1 cancer tumours found that looks can be deceptive.
The researchers deleted the BRCA1 gene in both mouse basal stem cells and luminal intermediate cells.
Tumours formed in both kinds of cell, but only luminal cells had features identical to human BRCA1 cancers. They also matched the majority of human basal-like "triple negative" cancers not associated with BRCA1 mutations.
Study leader Dr Matt Smalley, from the charity Breakthrough Breast Cancer, said: "These results represent a major advance in our understanding of breast cancer. It means we can now look very closely at where the disease forms and which genes are involved in that process."