The genetic clue behind why autism is diagnosed far more often in boys

Scientists may have cracked one of autism's biggest mysteries. Researchers at Baylor College of Medicine and Texas Children's Hospital have identified genetic variants in the regulatory regions of the MECP2 gene — located on the X chromosome — that may help explain why autism is diagnosed roughly four times more often in boys than in girls. Because females carry two X chromosomes, a second healthy copy can compensate for a mild mutation; boys, with only one X, have no such backup.

The team used a high-throughput lab tool called a Massively Parallel Reporter Assay (MPRA) to screen MECP2's regulatory regions and pinpoint variants that alter gene expression. They found that male autistic individuals had inherited certain variants from their unaffected mothers that subtly reduced MECP2 activity — enough to produce autism-like behaviors, but not the severe features of Rett syndrome.

Key Takeaways:

• One identified variant reduced MECP2 expression by ~30% in human neurons — a level linked to social deficits, hyperactivity, and anxiety in mouse models
• The affected individual carried an autism + ADHD diagnosis, consistent with the mouse model predictions
• This framework could guide discovery of regulatory variants in other X-linked neurodevelopmental genes, helping explain autism's "missing heritability"

Why it matters: Understanding the sex-based genetic mechanisms behind autism could reshape how clinicians screen for and diagnose the condition, and open new doors for targeted therapies.