Complex sentence: NSTX is quite similar to saxitoxin, like all the neurotoxins associated to PSP, the only difference is that NSTX shows one hydroxyl group bonded to nytrogen "1", where saxitoxyn contains one hydrogen (that group is in the red circle of the image).
Split into simpler sentences: NSTX is quite similar to saxitoxin, like all the neurotoxins asscociated to PSP, and the only difference is the addition of one oxygen to the nitrogen located at position "1". Indeed, as saxitoxyn contains an hydrogen bonded to nytrogen "1", NSTX shows an hydroxyl group in the same location (that group is in the red circle of the image).