Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. XAB2 has been shown to interact with ERCC8 and XPA. PRoteins (also known as polypeptides) are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. THe amino acids in a polymer are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues. THe sequence of amino acids in a protein is defined by the sequence of a gene, which is encoded in the genetic code. IN general, the genetic code specifies 20 standard amino acids; however, in certain organisms the genetic code can include selenocysteine and in certain archaea pyrrolysine.