Scientists have discovered how mutations in two key proteins may lead to autism.
Autism impairs social interaction
They have shown one protein increases the excitability of nerve cells, while the other inhibits cell activity.
The University of Texas team found that in normal circumstances the proteins balance each other out.
But the study, published in Neuron, suggests that in people with autism the balance between the proteins is knocked out of kilter.
The findings back the theory that autism involves an imbalance between excitatory and inhibitory connections between nerve cells.
The proteins, which serve to physically link nerve cells together, were discovered by the team at the university's Southwestern Medical Center more than a decade ago.
However, until the latest study their exact function had been unclear.
Researcher Dr Ege Kavalali said: "Mutations in these proteins have recently been linked to certain varieties of autism.
"This work provides clear insight into how the proteins function. We can never design a therapeutic strategy without knowing what these mutations do."
Bridge between cells
The proteins - neuroligin-1 and neuroligin-2 - create a physical bridge at the junction - or synapse - of nerve cells, enabling them to make connections with others.
In studies on rats the researchers showed that raising levels of both proteins in nerve cells led to the creation of extra synapses.
Neuroligin-1 was associated with excitatory connections