Drexler Technology Corp. based in Mountain View, Calif., has cautioned Microsoft Corp. of Redmond, Wash., that the software giant may be infringing on five of Drexler's US patents related to optical memory. Drexler claims that Microsoft has made extensive plans to enter the optical read-only memory card market. In its advisory letter, Drexler warned that if Microsoft continues its program the company could be in danger of violating five US patents and their counterparts in Canada, Korea, Mexico, Spain, Belgium, Germany, Israel and the UK. Drexler offered to sell nonexclusive licenses to Microsoft to use its optical read-only memory technology. Drexler Technology has received optical-memory-card-related patent license fees of more than $35 million to date.