"NetApp sued Sun last year and Sun counter-sued, saying NetApp infringed 22 of its patents. The Sun strategy is to invalidate the six NetApp patents by identifying prior use of the technologies involved (prior art), meaning that NetApp did not itself invent the technology. There have been two strands to this. One has been to ask the US Patent and Trademark Office (PTO) to re-examine the six NetApp patents and test their validity. In June it granted re-examinations on five of the six and invalidated one of them, leaving one down and five to go."