A simplified decision tree for predicting yeast FTase substrates. The output of Ydj1 EF scores was fit into a decision tree model considering each of the variable CaaX motif sites: a₁, a₂, and X. Our results strongly suggest that a major determinant of FTase activity depends on a select few discriminatory amino acids at a₂ (D, E, K, and R) and X (K, P, and R) positions, with the potential for certain allowable amino acids at a₂ (I, L, M, and V) that can mitigate discrimination caused at the X position. For the full decision tree, see Supplementary Fig. 5.