PCA plot of GPCR data set target space. (A) Shown are the best performing descriptor (Z-Scales (3) and Z-Scales (Avg)), and (B) the worst performing descriptor (ProtFP (Feature)). The (A) panel shows are more explicit clustering compared to ProtFP (Feature) in (B). The red circles indicate the histamine receptors and the black circles the muscarinic acetylcholine receptors. The lower performance of the histamine receptor family can be rationalized in both cases, as no clear clustering is apparent for this family. Conversely, both plots demonstrate clustering for the ACM receptors, which might explain the good performance.