Hi, Has anyone gotten negative expression values when using li.wong normalization by Affy? What the negative values mean? Here are what I did: > mouse <- ReadAffy() > liwong = express(mouse, bg=subtractmm, summary.stat=li.wong) > write.exprs(liwong,file="LiWong.txt") The min = -2358.292881, and max = 8763.940708. Guangchun