Many orphan sequences are being produced by human genome project. As the first step to the prediction of their structure and function, a software system SOSUI for discriminating membrane proteins from amino acid sequences alone was developed which is available on an internet web page. The analyses of amino acid sequence from total genomes by SOSUI system showed various interesting results, including the constant ratio (about 25%) of membrane proteins in total ORFs from bacteria to eukaryota. The physical background of the algorithm of SOSUI is discussed together with the results of the genome scale calculation.