Figure 1. Reverse transcriptase–polymerase chain reaction identified 641- and 370-bp fragments indicative of cDNA coding for carcinoembryonic antigen (CEA) and cytokeratin-20 cDNAs, respectively. Electrophoretic gel (top): Lanes 1 to 6 and lane 8, blood from seven patients with colorectal cancer showing CEA cDNA positive results for patients 3, 4, and 8; lane 7, CEA cDNA positive control (HT115 colorectal cell line); lane 9, CEA cDNA negative control (H₂O plus reagents). Electrophoretic gel (bottom): Lanes 1 to 3 and lanes 5 to 8, blood from seven patients with colorectal cancer showing cytokeratin-20 cDNA positive results for patients 1, 3, 5, and 6; lane 4, cytokeratin-20 cDNA positive control (HT115 colorectal cell line); lane 9, cytokeratin-20 cDNA negative control (H₂O plus reagents).