An acne-like rash is good news for colon cancer patients because it shows that a targeted therapy for the disease is working, Belgian scientists said on Thursday.

They found that the severity of the rash corresponded to the patient’s response to ImClone Systems Inc’s drug Erbitux and to the length of their survival.

“We have found there is a correlation between the activity (of the drug) and the skin rash,” Professor Eric Van Cutsem, at Gasthuisberg University in Leuven, Belgium, said in a telephone news conference.

Van Cutsem, who presented the findings to a medical conference in Geneva, tested the drug in combination with the cancer treatment irinotecan on 346 patients with advanced colon cancer which failed to respond to at least two previous chemotherapy regimes.

Erbitux, which is also known as cetuximab, works by blocking a protein called epidermal growth factor that is found on the surface of some cells and plays a role in regulating cell growth.

It is approved for use with colon cancer patients who have not responded to other treatments. The drug is also being studied as a treatment for head and neck cancer and as a first-line drug for colon cancer. The researchers do not know why the rash develops but said its severity is important because it is an indication of the efficacy of the drug.

Eighty-seven percent of patients in the study developed the skin condition but those who had the most serious rash had a median survival of 13 months, compared to 4.9 months for sufferers with a mild reaction.

The survival time for patients who did not suffer the side effect was 2.1 months.

“The more the severe the rash, the more efficient the treatment is,” said Van Cutsem. “We don’t have a clear explanation why but it is an important finding.”

Van Cutsem and his colleagues are doing further studies to learn more about the link between the rash and drug response.

Erbitux was at the center of an insider trading scandal that landed ImClone’s founder in jail and led to the conviction of U.S. lifestyle trendsetter Martha Stewart on charges of lying about a stock sale.