The CDATA type indicates that the attribute can contain an arbitrary string of character data. Character references (such as &) are recognized and expanded within the string, as are entity references. Attribute strings cannot contain unescaped markup characters (such as <, >, and &). Also, as in most programming languages, because the attribute value is quoted, it cannot contain unescaped quote characters (for example, "He said "Boo"" would not be valid). In some cases, using the alternative quote character (' instead of ") can be simpler than using the " built-in entity reference: 'He said "Boo"'.