Understanding why women experience depression at twice the rate of men could reshape how we approach mental health treatment across genders. This neurobiological puzzle has long confounded researchers who struggled to separate the effects of hormones from genetic sex differences. A breakthrough study using specially engineered mice reveals that sex chromosomes themselves—not just hormonal differences—directly influence how brain circuits respond to chronic stress. Researchers employed Four Core Genotypes mice, which allow unprecedented separation of chromosomal sex from gonadal hormones, to examine stress responses in corticostriatal brain regions critical for mood regulation. Under chronic unpredictable stress conditions, mice with XX chromosomes showed distinct neural vulnerability patterns compared to XY mice, regardless of whether they possessed ovaries or testes. The corticostriatal circuits, which connect the prefrontal cortex to the striatum and regulate emotional processing, exhibited sex chromosome-specific changes in gene expression and cellular stress markers. These findings challenge the prevailing assumption that sex differences in depression stem primarily from estrogen and testosterone fluctuations. Instead, they point to fundamental genetic programming that makes certain brain circuits inherently more susceptible to stress-induced dysfunction. This research represents a paradigm shift from hormone-centric explanations toward chromosomal mechanisms in psychiatric vulnerability. The implications extend beyond basic neuroscience to personalized medicine approaches. If sex chromosomes independently influence stress resilience, therapeutic strategies may need gender-specific targeting that accounts for genetic rather than purely hormonal differences. While conducted in mice, these findings provide a crucial foundation for understanding human sex differences in depression and could ultimately inform more effective, biologically-informed treatments for mood disorders.