This ENU-induced mutation was isolated in a screen at the University of Pittsburgh. The molecular lesion is a T to C substitution at coding nucleotide position 809 in exon 6 of the cDNA (c.809T>C, NM_019413). This changes the isoleucine residue to threonine at position 270 of the encoded protein (p.I270T). (J:175213) Additional incidental mutations were detected in sequencing for the causative mutation, Robo1b2b872Clo, and may be present in stocks carrying this mutation.