The protein encoded by Brsk1 is a serine/threonine-protein kinase that is involved in neuronal polarization, axon branching, and presynaptic differentiation in diverse neuronal populations. These mice carry a targeted mutation of the Brsk1 gene in which exon 1 is replaced by a floxed NEO cassette. Mice that are homozygous for the targeted mutation are viable, with reduced fertility but no overt phenotype. No gene product (mRNA) is detected by Northern blot analysis of brain tissue from homozygous animals. When crossed to Brsk2 knock out mice, or to Brsk2 floxed mice (Stock No. 023199) and Cre recombinase expressing mice, the resulting double null homozygotes die shortly after birth due to respiratory failure.