Abstract: This invention provides vectors comprising isolated polynucleotides comprising a member selected from the group consisting of: (a) a polynucleotide encoding MAB-21 protein; (b) a polynucleotide capable of hybridizing to and which is at least 60% identical to the polynucleotide of (a); and (c) a polynucleotide fragment of the polynucleotide of (a) or (b). This invention further provides host-vectors system for the expression of MAB-21 protein and protein products of mab-21 homologs. This invention further provides MAB-21 protein and protein products of mab-21 homologs. This invention also provides antibodies capable of specifically binding to the MAB-21 protein or protein products of mab-21 homologs. Finally, this invention also provides methods for identifying various suppressor, enhancer and modifier genes of the mab-21 and its homologs.