This document discusses protein motifs and domains. It defines a motif as a recurring arrangement of secondary structure found in multiple proteins, such as the HTH, HLH, and hairpin motifs. A domain contains one or more well-characterized motifs and has an independent function. Two common motifs are described: the HTH motif, which contains two antiparallel alpha helices connected by a beta turn for DNA binding; and the HLH motif, which contains two helices connected by a loop, with the larger helix binding DNA and the smaller helix aiding folding. Domains are defined as distinct functional units that are evolutionarily conserved and can exist independently; they are classified based on secondary structure composition.