Australian researchers have discovered a first for an amino acid: Proline can trigger mouse embryonic stem cells to differentiate into cells that form neural tissue. To find possible disease treatments and to learn more about embryonic development, biologists search for conditions that can coax stem cells to transform into different tissue types. Previously, scientists had found that mouse embryonic stem cells grown in a broth called MEDII become precursors to nerve cells. When Michael B. Morris of the University of Sydney and colleagues searched through MEDII’s thousands of molecules, they found two that were necessary for the transformation: a cytokine called leukemia inhibitory factor (LIF) and proline. Although LIF and proline are both required for the first step in differentiation, Morris reported that only proline could continue the process. The researchers have begun to pinpoint the cellular signaling pathways these molecules turn on and have found that they affect many of the same ones. Morris thinks that cross-talk between the LIF- and proline-activated cellular circuits are responsible for controlling the cells’ fate.