He was 92.
Officials at the University of Chicago said Jensen died of complications of pneumonia Dec. 16 in a nursing facility in Cincinnati, the Chicago Tribune reported.
In the 1980s Jensen, a molecular biologist, developed techniques for detecting and measuring estrogen receptor proteins in breast cancer, a procedure now used by pathology labs all over the world.
The presence of the protein indicates a tumor will respond to tamoxifen, a drug that blocks estrogen from binding with receptors in the tumor.
That treatment is much less toxic than the chemotherapy otherwise required to treat breast cancer.
"His work changed the way all breast cancers are analyzed," said Geoffrey Greene, who worked with Jensen in what is now the Ben May Department for Cancer Research at the University of Chicago.
Jensen retired from the University of Chicago at the then-mandatory retirement age of 70, but continued to do research with several other organizations before joining the University of Cincinnati College of Medicine in 2000.