I have no idea at all whether the following problem is known in the literature: I'm looking for codes where the code words are "non-stuttering", i.e. have no two consecutive identical letters. The specific parameters I care about are a 4-letter alphabet with code words at minimum distance 4, probably with words in the vicinity of 9-10 letters. (Yes, for the application I have in mind, the 4-letter alphabet is {A,C,G,T}.) I can think of all sorts of very bad constructions for codes with this obnoxious property, but not anything that seems clever. (The map from all strings on {1,2,3} to non-stuttering strings on {0,1,2,3}, taking successive sums, is frustratingly useless, since it is very badly behaved with respect to Hamming distance.) Can anyone offer insight or references? --Michael Kleber -- It is very dark and after 2000. If you continue you are likely to be eaten by a bleen.