Outline of experimental design. All four tumor RNAs plus the reference RNA were amplified and labeled twice with each dye, in both the Amsterdam and California laboratories. Half of the labeled material was exchanged between the two labs, and samples labeled locally and in the other laboratory were hybridized in replicate, and scanned. Slides were shipped to the other laboratory for rescanning. In the third lab (Paris), the tumor samples were independently labeled and hybridized three times.