SRSF3 binds to poison cassette exons in SR proteins. (a) SRSF3 and SRSF4 CLIP-tags and clusters around the alternative cassette exon of SRSF3 and SRSF7 genes. Labels as in Figure 1a. The zoom in represents the ultraconserved regions identified in [42, 43]. Note that the genes in the antisense strand run from right to left. (b) The enrichment of mRNAs encoding different SR protein family members after UV crosslinking and SRSF3 or SRSF4 immunoprecipitation (IP). To prime the RT reactions, hexamers were used. Data are presented relative to the input sample. IP is the specific immunoprecipitation and mock is the non-immune control. *P < 0.05, **P < 0.01, ***P < 0.001 (Student's unpaired t-test, n = 3-6). Error bars are standard deviation.