The floxed neo cassette was excised from Slit1tm1Matl via in vivo cre mediated recombination, leaving the endogenous locus disrupted by a cassette containing an endoplasmic reticulum KDEL retention sequence, a stop codon, and an IRES-tauGFP fusion protein. The initial insertion of the cassette replaced sequence encoding a portion of the second leucine rich repeat. In situ hybridization studies using a 3' antisense probe did not detect mRNA in the ventral spinal cord of homozygous mutant E11.5 embryos.