Dwarfism plays a key role in adapting crops to high-input production systems and contributes to improved lodging resistance and fertilizer efficiency.  The previously described dw3 and br2 dwarfing mutations of sorghum and corn, respectively, represent orthologous polar auxin transporter p-glycoproteins.  The recessive dw3 allele of sorghum is caused by an 882 base-pair tandem duplication in exon five that is unstable since the duplication can be lost through unequal crossing over during meiosis.  Tall revertant plants are termed ‘height mutants’ and are undesirable in seed and grain production fields.  The objectives of this research were to identify and characterize stable alleles of the dw3 locus.  To identify novel alleles of dw₃, PCR primers flanking the 882 base-pair tandem duplication that is diagnostic for this allele were used to screen a sorghum diversity panel containing 300 dwarf genotypes.  Accessions that did not contain the 882 base-pair tandem duplication were characterized by DNA sequencing to identify novel haplotypes in this highly conserved region of exon 5.  These experiments showed that 180 accessions have the 882 base-pair tandem duplication that defines the dw₃ allele and 71 accessions have a DNA sequence identical to the Dw₃ allele.  Numerous unique haplotypes that were distinct from either the Dw₃ or dw₃ allele were found in the remaining accessions.  Three accessions have an allele that is defined by a 6-bp deletion that results in a completely stable dwarfing trait.  At least 13 other DNA sequence variants were identified in this region of the gene and these alleles are being characterized to determine the effects of these alleles on plant height and stability of the trait.  These haplotypes are defined by 10 single-nucleotide polymorphism (SNPs), two deletions, and a small duplication. It is likely that several of the alleles defined by these haplotypes will produce stable dwarf phenotypes, especially the alleles with insertions and deletions in the conserved regions of the gene.  Near-isogenic lines of sorghum and maize contrasting for dw3 and br2 dwarfing mutations, respectively, are being used to compare and contrast the effects of these dwarfing alleles on growth and development of these close relatives.  Preliminary data suggests that the dwarf allele has several pleiotropic effects in these species.