Hi, The standart error returned by the se.exprs slot are for a given gene, the same on the different arrays. Is it correct ? or there is a bug in the se.exprs slot ? Example : data(Dilution) e <- express(Dilution) se.exprs(e)[1,] se.exprs(e)[1,] return 0.04187569 for all the arrays ... Thanks L. Buffat