We shorthand these as being of type 4, 2 and 1 respectively. A few sites are not classifiable into one of the above three classes -ATx codes for three isoleucines and one methionine and CGG and GGG are synonymous although one results from the other by a transversion. Treating, for example, ATG as a type 1 site and ATA, ATC and ATT as type 4 sites, means however, that the approximations made by us are most likely to be minor.