Clk/STY is a LAMMER protein kinase capable to phosphorylate serine/arginine-rich (SR) proteins that modulate pre-mRNA splicing. Clk/STY alternative splicing generates transcripts encoding a full-length kinase and a truncated catalyti-cally inactive protein. Here we showed that clk/STY, as well as other members of the family (e.g. clk2, clk3 and clk4),are up-regulated during HMBA-induced erythroleukemia cell differentiation. mRNAs coding for the full-length and thetruncated forms were responsible for the overall increased expression. In clk/STY, however, a switch was observed forthe ratio of the two alternative spliced products. In undifferentiated cells the full-length transcript was more abundantwhereas the transcript encoding for the truncated form predominated at latter stages of differentiation. Surprisingly,overexpression of clk/STY did not alter the splicing switch upon differentiation in MEL cells. These results suggest thatclk/STY might contribute to control erythroid differentiation by a mechanism that implicates a balance between thesetwo isoforms. Clk / STY is a LAMMER protein kinase capable to phosphorylate serine / arginine-rich (SR) proteins that modulate pre-mRNA splicing. Clk / STY alternative splicing generates transcripts encoding a full-length kinase and a truncated catalyti-cally inactive protein. Here we showed that clk / STY, as well as other members of the family (eg clk2, clk3 and clk4), are up-regulated during HMBA-induced erythroleukemia cell differentiation. mRNAs coding for the full-length and the truncated forms were responsible for the overall increased expression. In clk / STY, however, a switch was observed forthe ratio of the two alternative spliced ​​products. In undifferentiated cells the full-length transcript was more abundant whereas the transcript encoding for the truncated form predominated at latter stages of differentiation. , overexpression of clk / STY did not alter the splicing switch upon differentiation in MEL cells. These results suggest thatclk / STY might contribute to control erythroid different iation by a mechanism that implicates a balance between thesetwo isoforms.