Northern blot analysis of the candidate ES clones. (A) The exposed X-ray film of the Northern blot with a GFP cDNA fragment as the probe. U, Undifferentiated ES cell population. D, Differentiated ES cell population. TC1, wild-type ES cells. REX-1, an ES clone with the REX1::EGFP reporter transgene. 5C32, 5C18, 5C1, 5A3 and 2G2 are gene-trapped ES clones isolated in this study. (B) The RNA-agarose gel under an ultraviolet (UV) lamp. Approximately 10 μg of total RNA was loaded for the U and D samples of the control samples and each ES clone, except that 5 μg of total RNA was applied for the ES clone 5A3 and 2G2. The positions of 18S and 28S ribosomal RNAs on the agrose gel are marked.