To determine the residues of the aptamer which bind to hydrocortisone a folding and docking study was conducted using molecular dynamics simulations. Previously, Yang et al. have found that the cortisol aptamer binds hydrocortisone with its T AG residues in positions 42-44 of the sequence. Additionally, it was also shown that coritsol with its C 17 ketone hyroxyl hydrogen is available for a CH -π bond with the aptamer with some increase in positive charge on key hydrogens . It was also demonstrated that cortisol binds to this region with high affinity, leading to a conformational change from a pseudoknot structure to a one stem-loop structure . Hence we predicted this conformational change to cause a release of TO, resulting in a reduction in the fluorescent intensity. Figure shows the results of the docking study. On the left (figure ) the predicted secondary structure is shown having a stem-loop with a large hairpin structure. After folding this structure and docking the molecule to cortisol, the contact surface between the two molecules was determined at a maximal distance of 3 Å and computed a surface area of 235.8 Å 2 . The involved residues of the aptamer were determined as well and highlighted in blue in figure . The following residues were involved: C13, D37, T38, G39, A40, T42, A43, G44, C46. These results were in agreement with predicting high affinity binding of the residues T42, A43, G44 of the aptamer to cortisol. In the following, we analysed the binding of the aptamer and cortisol as well as the fluorescence intensity of thiazole orange to determine the predicted release of TO due to the binding of aptamer and cortisol.