Few have transformed cancer treatment on a global scale—because not many are like Mary Horowitz.
For more than three decades, Mary Horowitz, MD, MS, has transformed blood and marrow transplantation, leading research that expanded donor options, improved survival rates, and made transplants safer and more widely accessible. As the Chief Scientific Director Emeritus of the Center for International Blood and Marrow Transplant Research (CIBMTR), she helped turn what was once a high-risk procedure into a lifesaving standard of care.
She now serves as Deputy Director of the MCW Cancer Center and Robert A. Uihlein, Jr. Chair in Hematologic Research at MCW, driving initiatives to expand clinical trial access, advance transplant research, and strengthen the Center’s impact as a leader in cancer discovery and patient care. Recognized as one of the world’s top scientists, her work has shaped global health policies and given thousands of patients access to treatments once out of reach.
Yet, her impact extends far beyond the lab. A dedicated mentor, advocate, and leader, she has inspired generations of scientists and clinicians while forging deep connections with colleagues, patients, and friends. Whether championing better treatments, guiding early-career researchers, or simply sharing a laugh, Dr. Horowitz approaches everything with conviction, clarity, and a deep appreciation for human connection. “You gotta have laughs. That’s what gets you through,” she said.
An Unexpected Path and the Support That Made It Possible
Growing up as the oldest of seven in a traditional Irish-Italian family in Brooklyn, Dr. Horowitz learned early how to navigate chaos, solve problems, and challenge expectations. Her father didn’t believe women needed a college education, but that only made her more determined. “The best way to get me to do something was to tell me I couldn’t,” she said.
She initially set her sights on becoming a special education teacher, inspired by her experiences working with children with disabilities, including her own brother. But a single conversation in college changed everything. A graduate student in biology suggested she consider medical school, planting a seed of possibility. Then, a chance encounter with a molecular biologist—a woman who seamlessly balanced a thriving research career and a family—showed her that a different path was possible.
Still unsure, she took a semester-long teaching job in Milwaukee, hoping to confirm her calling. Instead, she discovered something even more life-changing—Mark, the elementary school teacher she would later marry. From that moment, everything aligned.
She enrolled in medical school at MCW, completed her internal medicine residency and hematology/oncology fellowship, and built a life in Milwaukee, raising three children while navigating two demanding careers.
But in the Horowitz household, balancing work and family was never up for debate; it was a shared commitment. “We never had to argue about who should do what. Whoever was there did what had to be done,” she said. Their 49-year marriage, lasting until Mark’s passing, became one of the strongest pillars of her success—a partnership built on mutual respect, unwavering support, and an unspoken understanding that they were always in it together.
Trial, Error, Breakthrough
Dr. Horowitz’s career-defining moment came unexpectedly—in a biostatistics class. While earning a master’s degree, she was assigned to analyze bone marrow transplant outcomes data. That project introduced her to Dr. Mortimer Bortin, the visionary founder of the CIBMTR. What began as a data analysis project soon became her life’s work. Within a few years, she was leading the organization as Scientific Director at just 37 years old.
At the time, bone marrow transplantation was an incredibly high-risk procedure—toxic, often fatal, and largely limited to younger patients with perfectly matched donors. Survival rates were low, and for patients over 40, transplants weren’t even an option.
Dr. Horowitz saw the power of data to change that reality. Under her leadership, the CIBMTR became the world’s largest and most influential data repository for blood and marrow transplantation. By bringing together many of the best minds in the field to analyze thousands of patient outcomes, the CIBMTR helped expand donor options, refine transplant protocols, and prove that older and sicker patients could safely undergo transplants. The work identified key factors—from graft-versus-host disease management to donor compatibility and post-transplant care—that dramatically improved survival rates.
In 2023, these collaborative research efforts didn’t just transform transplant medicine but changed national policy. A landmark Medicare decision expanded transplant eligibility for older adults with blood disorders, giving thousands of patients access to a treatment that had long been out of reach.
Dr. Horowitz’s leadership has always been rooted in a problem-solving mindset, one she applies to both science and life. “You take a problem, test possible solutions, and keep adjusting until you get it right,” she said. It’s this philosophy that has fueled decades of breakthroughs, transforming transplantation from a risky last resort into a widely accessible treatment that now saves lives worldwide.
“For years, we knew transplants could help these patients, but they simply couldn’t access them. Now, they finally can,” she said.
But beyond the data and discoveries, her work has always been guided by a deep empathy for patients. As the husband of a former patient once shared, “For all your research accolades, you were an amazing clinical doctor. Your humility, intelligence, and emotional intelligence were evident in every difficult interaction.”
Doctor, Patient, and Unshakable Optimist
Having spent her career advancing cancer care, Dr. Horowitz never expected to need it herself—twice. But facing breast cancer in 2003 and tonsillar cancer in 2018 only deepened her understanding of what patients go through. “Honestly, being a doctor made it easier. I knew what all the words meant. I knew I was getting the best care,” she said.
Her experience also deepened her appreciation for MCW, the institution she helped build. “I realized that no matter who you are, you’re treated with kindness and respect. That made me love where I worked even more.”
While she has always been a fearless advocate for her patients, public speaking never came naturally to her. Early in her career, she took beta blockers before big talks to keep her hands from shaking. Decades later, despite earning some of the highest honors in her field, she still over-prepares for speeches. But when she speaks, people listen.
Beyond research, Dr. Horowitz is one of the field’s most dedicated mentors, shaping the next generation of scientists and clinicians. “One of my greatest joys is seeing a mentee succeed—getting a paper published, receiving their first grant. Their success is even more rewarding than my own,” she said.
The Scientist Who Never Wanted the Spotlight, But Changed Cancer Care Forever
As she transitions into retirement, Dr. Horowitz remains deeply committed to cancer research and the Cancer Center’s pursuit of designation from the National Cancer Institute. “I hope we get it. We deserve it,” she said. “Cancer research and care in Milwaukee have been transformed, and it’s been incredible to be part of that.”
Despite all her accomplishments, she attributes much of her success to luck—a belief that underscores her humility. “I was in the right place at the right time. I fell into a job that completely suited my skills,” she added. Yet, anyone who knows her understands that luck had little to do with it. Her vision, persistence, and ability to bring people together have changed the course of cancer treatment worldwide.
Her advice to young professionals is simple but profound: “You know you’ve done the right thing when you’re happy going to work in the morning and happy going home at night,” she said.