Turning off a gene early in mouse development led researchers to end up with an accidental six-legged embryonic mammal.

While legs and arms share many of the same genes, this early in the process, the hindlimbs and genitals have more in common. There has been some suggestion they arise from the same initial primordium structure in ancestral species.

The researchers still don’t know the exact mechanism between the Tgfbr1 gene suppression leading to an extra pair of legs.

Understanding more about these fundamental processes will equip researchers with additional tools to tackle developmental challenges and diseases.