Compounds studied in neurological and signalling contexts, including short synthetic peptides built on endogenous fragments and ligands selected for receptor selectivity.Selectivity is the recurring theme here. Within a receptor family whose subtypes sit in different systems, how broadly a ligand engages them determines how many systems it touches — which is the entire distinction between otherwise similar compounds. Our guides on Semax and Selank and on melanocortin receptor selectivity work through two cases where the distinction is routinely lost.All products are sold for laboratory research use only.