Scientists believe they have found a way to spot which breast and ovarian cancers are likely to be aggressive.
It can be hard to predict outcomes
A team from the University of Texas, in the US, found cancer cells that overexpressed a particular protein were more likely to be fatal.
Low levels of the protein in ovarian cancer was linked with an 80% chance of surviving for five years compared to a 50% chance with high levels.
The findings, based on a study of 100 women, appear in Nature Medicine.
They matched tumour samples to outcomes in women diagnosed with either breast or ovarian cancer.
They found that a low level of a protein called Rab25 in the tumour sample was associated with a better clinical outcome in both types of cancer.
In breast cancer, the chance of surviving five years after treatment was 60% if Rab25 levels were low, but was only 40% if Rab25 levels were high.
The scientists are hopeful that adding this protein to other known markers of cancer progression could enable doctors to make accurate predictions of outcomes for women with breast and ovarian cancers.
Not only that, but the protein might offer a way of treating these cancers.
Lead author Dr Gordon Mills said: "We are pursuing Rab25 both as a test for outcomes and as a possible treatment."
He said experimental drugs based on blocking the function of the protein were being tested.
But he said: "We have a long way to go to understanding exactly what it is that Rab25 is doing, and how we might be able to use it in treatment."
Cancer Research UK funded biochemist Dr Jim Norman, from the University of Leicester, said: "This study is extremely exciting.
"This indicates that Rab25 could be used in the clinic as a marker of tumour aggressiveness and patient outcome.
"It also earmarks the Rab25 and Rab11 proteins and the membrane recycling pathways they control as targets for anti-cancer drug design."
Karen Scanlon from Breast Cancer Care said: "We feel that more research is needed to fully understand the workings of the Rab25 protein before it can be used as a predictive test of survival for women with breast or ovarian cancer."
Breakthrough Breast Cancer echoed this caution.