The Molecular ACCess System (MACCS) structural key fingerprint (FP) of a molecular graph is a bit vector whose entries indicate the presence (1) or absence (0) of a list of F = 166 pre-defined subgraph patterns (molecular substructures/fragments) . The number of "on" (1) bits in the MACCS FP is equal to the number of these subgraphs with presence in the molecule. The list of molecular patterns defining the MACCS feature map φ MACCS (G) : G → R 166 were curated by a company, Molecular Design Limited, Inc. (MDL), for drug discovery tasks . We use the MACCS fingerprint implementation in RDKit , whose source code lists the subgraph patterns (described by SMARTS strings) corresponding to each keybit . For example, keybits 29, 134, 125, and 154 indicate the presence of phosphorus, a halogen, an aromatic ring, and a carbonyl group, respectively. Fig. illustrates further. The fixed-length random walk feature vector contains counts of label sequences encountered along all fixed-length walks on a molecular graph. The table lists all label sequences encountered along length L = 2 walks on the molecular graph of 2,6-dichlorobenzonitrile. The numbers in the second column, giving the number of walks having each label sequence, comprise the nonzero entries of the random walk feature vector of 2,6-dichlorobenzonitrile.