BDNF and Cowden syndrome 1: Lacking a gold standard method to identify CS, the presence of symptoms of CS can objectively be evaluated using biomarkers containing brain imaging, such as assessing reduced gray matter volume, alterations in resting state functional connectivity, and lab results, such as altered levels of brain-derived neurotrophic factor (BDNF) and gamma-aminobutyric acid (GABA) [38,39,40,41].