We took advantage of the availability of the M. trunca- tula genome sequence [46] to develop high-density SNP (single nucleotide polymorphism) markers for fine map- ping of the Mt-NS1 locus. SNPs were genotyped either by converting to CAPS (cleaved amplified polymorphic sequences) markers or by direct sequencing. Phenotyp- ing and genotyping a total of 3,900 F2 plants using the SNP markers allowed us to delimit the Mt-NS1 locus be- tween SNP103 and SNP127 (Figure 3B), which span ~50 kb based on the genome sequence of Jemalong A17 (Nod + Fix-). The 50-kb genomic sequence of Jemalong A17 contains at least six predicted genes based on the Medicago truncatula genome release version 4.0 (http://www.jcvi.org/medicago) (Figure 3C).