Identification of Clk1/4 intron 3/4–retaining RNAs. (A) Schematic illustration of the intron-retaining RNAs of Clk1 and Clk4. Open boxes indicate constitutive exons, and gray boxes indicate selective exons. Positions of Northern blot antisense probes (bars) and PCR primers (arrows) used in the following assays are indicated. (B) Tissue expression profile of Clk1 pre-mRNA and mature mRNA in Northern blot analysis. mRNAs were detected by radiolabeled antisense probes. Lanes: 1, heart; 2, brain; 3, spleen; 4, lung; 5, liver; 6, skeletal muscle; 7, kidney; 8, testis. The asterisk shows that a faint band at 2.4 kb of lane 5 of the intron 3 probe is nonspecific. Gapdh was used as a control. (C) RT-PCR analysis of Clk family genes. Almost the entire regions of Clk1, 2, and 4 were amplified by RT-PCR from mouse embryonic (E) or adult (A) brains. The arrowheads and arrows indicate the mature and the premature mRNAs, respectively.