Illustration of targeted TM locus and Southern blot of TM-null ES cells. (A) Diagram of the targeted TM gene indicating the 5′ upstream region of the TM promoter (TM 5′), 153 bp of untranslated TM sequences (▪), the neo expression cassette used to replace the TM coding region (neo, ▨), and 3′ TM untranslated sequences (TM 3′). BglII restriction enzyme sites are indicated at B. (B) Southern blot analysis of ES-cell genomic DNA. Genomic DNA was isolated from wild-type ES cells (lane 1), from TM^+/− ES cells (lane 2) and from TM^−/−ES cells (lane 3) and hydrolyzed with BglII restriction enzyme. The hydrolyzed DNA was electrophoresed, blotted, and hybridized with a³²P-labeled cDNA probe against DNA sequences within the TM 3′ region. Markers are indicated in kb.