Figure 1: Structure of hnRNPs with multiple modules. Except hnRNP E, K, and U, all other reported hnRNP family proteins contain one or more RRM domains, the structural base responsible for the RNA/ssDNA binding. Instead of RRM domain, hnRNP E and K contain three copies of KH domains. Since KH domain displays relatively weaker RNA/ssDNA binding affinity, it is believed that several copies of KH domains within a given protein are required for achieving greater RNA/ssDNA binding affinity and specificity. RGG repeats domain is the only RNA-binding domain identified in hnRNP U responsible for RNA/ssDNA binding.