The US computer software industry involves about 60,000 companies with combined annual revenue of about $240 billion. About 60 percent of industry revenue comes from software publishing; custom programming accounts for the remainder. Large companies include Adobe Systems, CA, Microsoft, Oracle, and Symantec. The US packaged software segment is concentrated: the 50 largest companies generate about 70 percent of revenue. The US custom programming segment is highly fragmented. There are four general types of computer software which includes system software, middleware, application software, and engineering software. Many of the major software companies have been consolidating or acquiring other companies to provide products or services in multiple fields. For instance, rising demand for open source and On-Demand software, has led many larger companies to acquire smaller companies that specialize in these services.