This phenotypic allele was identified in an ethylmethanesulfonate (EMS) mutagenesis screen for neurological phenotypes. The molecular defect is a 5 base pair deletion between nucleotides 30 and 34 in a highly conserved region located in the U2 consensus branch site sequence (BSRS) and the linker region between the BSRS and the U2/U6 helix.