Isolation of representational difference analysis (RDA) differential products from single-cell microdissected archival breast tissues. RDA hybridization was of the normal cell DNA versus the metastatic tumor cell DNA and was performed in two ways. In the first RDA (lane 2) the metastatic cell DNA was used as the tester (which should yield differential sequences gained during malignant transformation or in the process of becoming metastatic), and in the second RDA (lane 3) normal DNA was used as the tester (which should yield sequences that were lost from the metastatic cells). (a), (b) and (c) Three rounds of RDA hybridizations were performed. In the third round of hybridization, the 'gain' lane contained four DNA bands (faint fourth band) ranging from 200 to 300 bp in size (c, lane 2) whereas in the 'loss' lane there are five bands (faint fourth band) ranging from 200 to 370 bp (c, lane 3) in size (arrows). Lane 1, low molecular weight DNA marker (2 kb).