A study investigating the function of AXL RTPKs in the adult rat brain suggested a role in neuronal survival and growth, and regulating synaptic function and plasticity.39 Further research is required to determine the extent to which these RPTKs regulate the molecular mechanisms of depression, however, the AXL gene identified as a biomarker for depressive symptoms in elderly individuals.40 The gene discussed is AXL; the disease is major depressive disorder.