lsy-20 is an uncloned gene required for complete developmental differentiation of ASEL neurons from ASERs, with lsy-20 mutants showing ectopic expression of the ASER-specific gene gcy-5 in ASEL, but with ASEL continuing to normally express lim-6; on the basis of its phenotype, lsy-20 is expected to act in concert with LIM-6 during the lateral specification of ASER/ASEL.