Thousands of people who have seen their cancers return could benefit from new drugs in the future after scientists made a breakthrough in understanding why the disease comes back.
Many people with lung, stomach, skin, bladder, pancreatic and ovarian cancer appear to beat the disease after receiving a common type of chemotherapy called platinum treatment.
However, frequently their cancer returns after they appear to develop resistance to the drug.
Sometimes they are put on other drugs - termed second-line therapies - although these often do not perform as well.
But now scientists at the charity Ovarian Cancer Action have discovered the reason why their cancers apparently develop this resistance.
Rather than the cancers developing immunity, they found that minute traces of cancers that were always resistant to platinum therapy were there from the beginning.
This discovery has helped them identify four or five different molecular "targets" that could be the focus of new drugs, said Prof Hani Gabra, director of the charity's research centre.
He said: "These cancers look like they are platinum-resistant, but in fact they were there from the outset and they were never touched by the drugs."
Unaffected, they had simply taken their time to grow, he explained.
That step-change in understanding meant they were able to concentrate on what exactly was different about the tumours which appeared later.
One of the targets was a gene called STAT1 that was "very active" in cells platinum-resistant cancer cells. They also found that an enzyme, called HDAC4, spurred it into action.
Sometimes genes and enzymes have already been studied for their role in different diseases.
The most exciting aspect, said Prof Gabra, was that a couple of these targets had already been studied and that drugs were consequently already in development. This eliminated the need for very early-stage research.
He said: "All we've got to do is put these drugs together with platinum treatment in a trial in people. We are hoping to do proof of concept trials this year or next year."
The study is published this month in the journal Cancer Research.