Hi, All; I simply upgraded my R from version 2.5.0 to 2.5.1 and the whole problems disappeared. So I am closing the question myself. It could have been the R version problem. Tae-Hoon Chung Post-Doctoral Researcher Computational Biology Division, TGEN 445 N 5th St. Phoenix, AZ 85004 USA O: 1-602-343-8724 F: 1-602-343-8840 [[alternative HTML version deleted]]