A recent article by the CAZy team defines subfamilies within the large GH5 family.
Details appear in Aspeborg H, Coutinho PM, Wang Y, Brumer H, Henrissat B (2012) Evolution, substrate specificity and subfamily classification of glycoside hydrolase family 5 (GH5). BMC Evol. Biol. 12:186 [PMID : 22992189]. The GH5 subfamilies now appear online at URL http://www.cazy.org/GH5_subfamilies.html