To provide a framework for studies to understand the contribution of SALT OVERLY SENSITIVE1 (SOS1) to salt tolerance in Thellungiella halophila, we sequenced and annotated a 193-kb T. halophila BAC containing a putative SOS1 locus (ThSOS1) and compared the sequence to the orthologous 146-kb region of the genome of its salt-sensitive relative, Arabidopsis thaliana. Overall, the two sequences were colinear, but three major expansion/contraction regions in T. halophila were found to contain five Long Terminal Repeat retrotransposons, MuDR DNA transposons and intergenic sequences that contribute to the 47.8-kb size variation in this region of the genome. Twenty-seven genes were annotated in the T. halophila BAC including the putative ThSOS1 locus. ThSOS1 shares gene structure and sequence with A. thaliana SOS1 including 11 predicted transmembrane domains and a cyclic nucleotide-binding domain; however, different patterns of Simple Sequence Repeats were found within a 540-bp region upstream of SOS1 in the two species. © 2009 Elsevier Inc. All rights reserved.