Hello. I am trying to find the IC50 values for some enzyme inhibitory-molecules. However, I was not able to find the 100% or 0% inhibition for some of them. I always used GraphPad Prism to calculate the IC50 - [inhibitor] vs response (three parameters). My question is: when I try to obtain these values for molecules that do not reach the 100% or 0%, should I normalize it? I understand that yes, I should, since GraphPad site says "It is the concentration required to bring the curve down to a point halfway between the "Top" and "Bottom" plateaus of the curve. Unless the data are normalized to run from 0 to 100, this is not the value that corresponds to 50% on the Y axis" about the Relative IC50. Also, should I use the three or four parameters equation? I understand that, if my low concentrations reach zero or close to zero, I should use the three parameters. Otherwise, I should use the four parameters. However, I am not sure and would appreciate the community's help.