Researchers have identified a compound that blocks the activity of a tumor cell enzyme and stops the growth of certain types of breast cancer in mice.

Targeted cancer therapies already allow doctors to shut down the growth of tumors with little or no harm to normal cells.

Several of these treatments are used for breast cancer with drugs “customized” to act on specific types of tumor cells.

Researchers now have set their sights on a number of breast cancers that currently have no targeted cancer treatment available. In a new study, researchers identified a compound that can block the activity of an enzyme involved in stimulating the growth of breast cancer.

In the study, which was published online today in Science Translational Medicine, an experimental compound — known as SR-3029 — stopped the growth of breast cancer tumors in mice, with few side effects.

“We saw no gross adverse effects with long-term daily dosing,” Derek Duckett, PhD, an associate professor of molecular therapeutics at The Scripps Research Institute, told Healthline. “We have yet to perform detailed toxicity studies.”

Read More: Researchers Find ‘Doorway’ That Allows Breast Cancer to Enter the Bloodstream »

The researchers tested the compound in several mouse models. These simulated different types of hard-to-treat breast cancers that affect people.

This included “triple negative breast cancers,” which account for about 10 to 20 percent of breast cancers. This group of cancers is named because their growth is not driven by estrogen, progesterone, or the HER2 gene — all of which have targeted treatments.

The researchers also tested the compound on tumor tissue collected from breast cancer patients. The results were similar.

More research is needed before this compound can the join the ranks of other targeted breast cancer treatments. Researchers will need to test the compound on a wider range of breast cancer tissue samples from patients. They will also need to develop another version of the compound suitable for clinical testing in people.

Read More: Safer, Quicker Breast Cancer Treatment Gets a Boost »

In the study, the researchers confirmed the compound SR-3029 works by blocking the activity of an enzyme called casein kinase 1, delta — or CK1delta. This enzyme promotes tumor growth by activating another protein, known as beta-catenin.

When the enzyme is blocked, the cancer cells are starved of beta-catenin. In the end, this kills the tumors.

The compound is “very specific” for the enzyme, Duckett said, which made it highly effective at stopping tumor growth.

This enzyme is also involved in many activities in noncancerous cells, such as regulating the body’s internal clock and helping cells maintain their shape.

It had been implicated in cancer before, but its role had not been clear until now.

Read More: New Imaging Technique Reveals Breast Cancer in Less Than 10 Minutes »

Targeted therapies exist for many types of cancer. The development of new ones depend on the ability of researchers to identify good targets — such as CK1delta.

Many types of treatments have been approved. This includes ones like SR-3029 that block part of a cellular pathway. But targeted therapies using hormones or immune system antibodies are also effective for certain cancers.

Scientists once thought that targeted therapies would be less toxic because they are aimed at specific types of cancer cells. But even these treatments can have major side effects, such as diarrhea and liver problems.

In addition, although targeted cancer therapies can be highly effective, cancer cells can become resistant to the drugs. To keep this from happening, these treatments are sometimes used with other targeted therapies or traditional chemotherapy drugs.

Despite the limitations of targeted cancer therapies, they remain a powerful tool for doctors in the treatment of cancers. They are also part of an overall push toward more personalized medicine.