Complex sentence: The selectivity of SSRIs for SERT is really interesting, only one or two different functional group substitutions are sufficient to convert an SSRI into a norepinephrine reuptake inhibitor (NRI) with higher affinity to norepinephrine.
Split into simpler sentences: The selectivity of SSRIs for SERT is really interesting. Only one or two different functional group substitutions are sufficient to convert an SSRI into a norepinephrine reuptake inhibitor (NRI) with higher affinity to norepinephrine.