In a landmark decision, the FDA approved Merck‘s (NYSE:MRK) immunotherapy Keytruda yesterday as the 1st cancer treatment indicated for any solid tumor with a specific genetic feature, regardless of tissue type.

Traditionally, the location where a cancer originates in a patient’s body determines the course of treatment. The move to treat cancers based on genetic biomarkers is a nod towards precision medicine, which physicians hope will help identify patients that are most likely to respond to specific therapies.

“Until now, the FDA has approved cancer treatments based on where in the body the cancer started – for example, lung or breast cancers,” Richard Pazdur, head of oncology products for the FDA’s Center for Drug Evaluation and Research, said in prepared remarks. “This is an important first for the cancer community,”

The accelerated approval was for solid tumor cancers with a biomarker called microsatellite instability-high or mismatch repair deficient. About 4% of advanced cancers carry the genetic traits addressed in this approval, according to the study’s lead investigator.

The new indication covers kids and adults whose cancer has progressed despite treatment and patients with no satisfactory alternative treatment options.

Tumors with the aforementioned genetic biomarkers are often associated with colorectal, endometrial and gastrointestinal cancers. Patients whose tumors are marked with the genetic defects are overloaded with abnormal proteins that trigger immune cells to seek out and destroy the cancer cells.

Keytruda is 1 of 5 drugs called PD-1 or PD-L1 inhibitors, which block tumors from evading detection by cancer-killing cells. Merck’s Keytruda was previously approved to treat advanced melanoma, advanced non-small cell lung cancer, classical Hodgkin lymphoma and head and neck cancers. Just last week it won an FDA nod for bladder cancer.

The company is still waiting on the regulatory watchdog’s decision regarding an indication for gastric cancer.

Dr. Drew Pardoll, principal investigator and director of the Johns Hopkins Bloomberg-Kimmel Institute, called it “a marriage of cancer genetics and cancer immunology.”

“This was 1 of those eureka trials where it didn’t take a lot of patients to see this was going to be something major,” Pardoll reportedly said in an interview.

Material from Reuters was used in this report.