Dr. John Emanuel Markis (Marnierakis) of Waban and Wellfleet, Massachusetts, Atsipopoulo, Crete and Martins Ferry, Ohio, son of the late Emanuel and Mary Markis, passed away on May 24, 2025, after a long illness. John is survived by his former wife, Carolyn Markis, their three children, Mary Markis of Chicago, IL, Emanuel (Donna) Markis of Wellesley MA, and Anastasia (Neil) Horne of Westfield, NJ, and six beloved grandchildren: John, James and Mary Markis, Katherine Kraczkowsky, and Julia and Natalie Horne. A devoted father and grandfather, John was also a dedicated physician and pioneer in the field of interventional cardiology.

Born during the Great Depression as the only child of Cretan immigrants, John's upbringing in Martins Ferry, Ohio was a lesson in hard work. At a young age, he delivered newspapers and later worked on a loading dock to save for his future. While not carefree, John's childhood fostered in him a lifelong love of nature and animals, as well as immense pride in his Cretan heritage and deep faith as a Greek Orthodox Christian. He proudly served as an Altar Boy, sang in church choir, and was a longtime member of the Pancretan Association.

John graduated from Martins Ferry High School in 1955 and put himself through The Ohio State University ('59) and The OSU College of Medicine ('66). Graduating in the top of his class, he was elected to the Alpha Omega Alpha Honor Medical Society and accepted into Harvard Medical School's highly competitive Boston City Hospital internship program as part of the II and IV Medical Services.

After completing his internship and residency, John continued what became a more than 50-year relationship with Harvard Medical School, embarking on a research fellowship in pharmacology. In 1970, he entered the United States Army, honorably serving two years as a Major in the Army Chemical Corps, earning the National Defense Service Medal. When he returned to Boston, he began his second Harvard fellowship in Cardiology at the Peter Bent Brigham Hospital. Throughout the 1970s and for the remainder of his career, John continued at Harvard, becoming an Instructor in Medicine in 1974, Member of the Faculty in 1975, and Assistant Professor in 1978. During these years he also served as Assistant Director of the Brigham Cardiac Catheterization Lab, later becoming Director of the Cardiac Catheterization Lab at Beth Israel Hospital. He remained full time at the Beth Israel Deaconess Medical Center, was Director of their ECG Laboratory, and maintained a private cardiology practice on Longwood Avenue until his retirement in 2018.

John once wrote that he "had the pleasure of being in interventional cardiology at an exciting time," when he played a pivotal role in changing the way heart attacks were treated. In 1981, John and fellow researchers pioneered the use of a clot-busting agent that, if administered within the first hours of a heart attack, could rescue heart muscle from death. The Boston Globe reported that this study, published as a lead article in The New England Journal of Medicine, provided "the first firm evidence that dissolving blood clots during heart attacks does rescue dying heart tissue." This study helped lay the groundwork for a paradigm shift that would significantly improve heart attack survival rates. It further played a key role in the establishment of the Thrombolysis in Myocardial Infarction (TIMI) Study Group. As a Principal Investigator in TIMI I and TIMI 2, John and his team developed the TIMI perfusion grading system, a system still widely used in judging the angiographic grade of perfusion in coronary interventions. John was also involved in Beth Israel's first endomyocardial biopsies, early percutaneous intra-aortic balloon placements, ergonovine challenges, and using intravenous nitroglycerine (which, at that time, had to be "home brewed" by crushing tablets and putting them through micropore filter). John authored more than 40 papers in top publications that, decades later, continue to be cited, including a 1976 paper that outlines a coronary arterial ectasia classification system — now referred to as "Markis Classification."

John was a Renaissance man, intellectually curious and always learning. John inherited from his mother a vast knowledge of plants and gardening, and would often surprise his children by calling birds down from the trees. Always the favorite of the family pets, John was deemed "the cat whisperer," a well-earned moniker as even his neighbor's cats lingered at his doorstep. He was also a masterful cook—renowned for his perfect popovers—and an avid fly fisherman who tied his own flies. He shared his lifelong passion for British cars with his son, his proficiency for languages and love of travel with his oldest daughter, and his appreciation of art and nature with his youngest. John was tremendously proud of his six grandchildren and their many accolades and accomplishments.

John's legacy of hard work and dedication will live on in his children and grandchildren, who will continue to be inspired by his unparalleled work ethic, his unwavering desire to help others, and his deep faith. They will forever miss his hearty laugh and his unconditional affection.

A wake will be held at St. Demetrios Greek Orthodox Church, 57 Brown Street in Weston, Massachusetts, on Saturday, May 31 from 5 to 7 pm, followed by a funeral service at The Orthodox Christian Church of the Life-Giving Fount/Zoodohos Peghe, 314 N. 5th Street in Martins Ferry, Ohio, on Tuesday, June 3 at 11 am. Memorial donations may be made to The Holy Transfiguration Monastery, 278 Warren St, Brookline, MA 02445.