Clk4 is a dual specificity kinase acting on both serine/threonine and tyrosine-containing substrates. Clk4 phosphorylates serine- and arginine-rich (SR) proteins of the spliceosomal complex and may be a constituent of a network of regulatory mechanisms that enable SR proteins to control RNA splicing. Clk4 phosphorylates SRSF1 and SRSF3. Clk4 is required for the regulation of alternative splicing of MAPT/TAU. Clk4 regulates the alternative splicing of tissue factor (F3) pre-mRNA in endothelial cells.