In a laboratory at the University of North Carolina at Chapel Hill, something unexpected happened that is changing how scientists think about hearts, sex, and survival. Researchers there have discovered that female hearts depend on a special protein that male hearts simply don't need. Remove that protein, and female hearts fail. Leave it intact, and hearts keep beating normally.
The protein is called DDX3X. It sits on the X chromosome, one of the two chromosomes that determine biological sex. Working with mice, the UNC team found something striking: when they deleted DDX3X from developing heart muscle cells, female embryos died mid-pregnancy from heart failure. Their male littermates, carrying the exact same genetic deletion, developed normally and survived to birth.
"The same genetic change produced two completely different outcomes depending on sex," said Kayla K. Mason, the study's lead author. "For the female heart, DDX3X is essential. For the male heart, it's dispensable."
So what does DDX3X actually do? Genes carry the body's instruction manual, but those instructions must still be read and copied before proteins can be built. Mason and her colleagues discovered that DDX3X acts at this copying stage. It unwinds tight knots in certain genetic messages, allowing the cell's protein-making machinery to do its job.
Female hearts naturally carry more DDX3X than male hearts because the gene escapes a process that normally silences one X chromosome in females. That extra dose appears to be exactly what female hearts need to build proteins essential for proper heart function—including proteins that control heart muscle assembly and the electrical signals that make a heart beat.
When DDX3X was removed, production of these critical proteins collapsed. The female embryos' hearts developed disorganized muscle fibers, beat erratically, and ultimately failed.
The findings connect to real human conditions. In Turner syndrome, where girls are born with only one X chromosome, roughly 90% of live births involve congenital heart disease. In DDX3X syndrome, a disorder caused by mutations in this gene, heart defects appear almost exclusively in girls—matching the pattern the researchers observed in mice.
"This is a new paradigm for thinking about sex differences in the heart," said Frank L. Conlon, senior author and professor at UNC's McAllister Heart Institute. He called the work a reframing of where scientists should look for the origins of heart disease that affects one sex more than another.
For millions of people, this discovery offers a new understanding of why heart disease can behave differently in women and men—a question researchers have struggled to answer for decades.