<?xml version="1.0" encoding="UTF-8" ?>
<!-- simple protein.dtd -->

<!ELEMENT PROTEIN (GENE?, INTERACTION*)>
<!ATTLIST PROTEIN name CDATA #REQUIRED
                  length CDATA #IMPLIED>

<!ELEMENT GENE EMPTY>
<!ATTLIST GENE name CDATA #REQUIRED>

<!ELEMENT INTERACTION (PROTEIN,GENE)>