KH binds single-stranded RNA or DNA. It is found in a wide variety of proteins including ribosomal proteins, transcription factors and post-transcriptional modifiers of mRNA. There are two different KH domains that belong to different protein folds, but they share a single KH motif. The KH motif is folded into a beta alpha alpha beta unit. In addition to the core, type II KH domains (e.g. ribosomal protein S3) include an N-terminal extension and type I KH domains (e.g. hnRNP K) contain a C-terminal extension. Some KH-I superfamily members contain a divergent KH domain that lacks the RNA-binding GXXG motif. Some others have a mutated GXXG motif which may or may not have nucleic acid binding ability.