May function in cell specification, particularly in the CNS. Interacts with HIPK1, HIPK2, and HIPK3. Expression detected in adult brain, testis and spleen. In the testis, expressed in the germ cells of the seminiferous epithelium, in elongating spermatids and in spermatozoa. Expressed throughout the brain with highest levels in regions of the cerebral cortex, hippocampus, diencephalon, pons, medulla and cerebellum. In the embryo, expressed in the developing posterior central nervous system. First seen in the ectoderm lateral to the primitive streak, later it encompasses the neural plate. Starting at day 9.5 pc, expressed in distinct areas of spinal cord, hindbrain, midbrain and forebrain. In the embryo, expressed at highest levels at day 10 with levels decreasing during further development. Phosphorylated by HIPK2 in vitro. Belongs to the NK-1 homeobox family. Probable cloning artifact. Aberrant splicing.