Northwestern University researchers have discovered that a protein, Lefty, produced by human embryonic stem cells (hESCs) can inhibit the growth and spread of breast cancer and malignant melanoma.

Similarities between stem cells and tumors–both are self-renewing and have the capacity to give rise to different cells types–previously led the researchers to find the protein Nodal, which facilitates cell growth, and suggested that stem cells must have a way to control Nodal.

The Northwestern researchers found that was Lefty. When aggressive tumor cells were exposed to the chemical environment of hESCs, which contained Lefty, their Nodal production fell sharply, and the tumor cells became less invasive and even started to die.