CDKL5 is a member of Ser/Thr protein kinase family and encodes a phosphorylated protein with protein kinase activity. Mutations in this gene have been associated with X-linked infantile spasm syndrome (ISSX), The STK9 cDNA predicts a protein product of 1,022 or 1,030 amino acids, depending on which initiating methionine is used. Northern blot analysis of human and mouse tissues detected a transcript larger than 9.5 kb expressed in a variety of tissues, and an abundant 3.5-kb transcript expressed only in testis.STK9 RNA isoform containing exons 1a and 1b (isoform II) is transcribed at very low levels in human fetal brain and testis but not in lymphoblastoid cell lines, whereas the STK9 RNA isoform containing exon 1 is expressed in a wide range of cells, including human fibroblasts and lymphoblastoid cell lines.