The product remains bound to farnesyltransferase until displaced by new substrates. The last three amino acids of the CaaX motif are removed later. There are four binding pockets in FTase, which accommodate the last four amino acids on the carboxyl-terminus of a protein. Only those with a suitable CaaX motif can bind ('C' is Cysteine, 'a' is an aliphatic amino acid, and 'X' is variable). The carboxyl-terminal amino acid (X) discriminates FTase's targets from those of the other prenyltransferases, allowing only six different amino acids to bind with any affinity. It has been shown that geranylgeranyltransferase can prenylate some of the substrates of Farnesyltransferase and vice versa. As of late 2007, 15 structures have been solved for this class of enzymes, with PDB accession codes , , , , , , , , , , , , , , and .