Babesia bovis is an apicomplexan parasite transmitted by ticks that causes bovine babesiosis. A live attenuated vaccine against babesiosis is available in many countries but its use is prohibited in United States (US) due to inherent risks. The attenuation of the vaccine is acquired by serial passages in splenectomized animals. The molecular mechanisms behind B. bovis attenuation are poorly understood. Genome comparison to the virulent strain showed a reduction of gene diversity in the attenuated strains. Subsequent transcriptome analysis indicated up-regulation of spherical body proteins 2 truncated copy family members (sbp2t). Thus, we hypothesized that sbp2t7, sbp2t9 and sbp2t11 are attenuation markers for B. bovis. Transcriptome comparisons among four different geographical B. bovis strains rejected sbp2t7 and sbp2t9 as attenuation markers but demonstrated that sbp2t11 is a consistent attenuation marker. We characterized the sbp2t11 gene and protein. The transcriptional profile of sbp2t11 showed that the transcript is down-regulated in the kinete stage and is solely expressed in t blood stages. It was also determined that SBP2t11 is a 30 kDa protein with a PEXEL-Like motif (PEXEL) which is cleaved into a 17 kDa fragment. Furthermore, we demonstrated a correlation between up-regulation of sbp2t11 transcript and over-expression of the protein in attenuated B. bovis. As part of the project, a Texas virulent-stably transfected B. bovis line with artificial up-regulation of the sbp2t11 gene was developed in order to mimic the regulation pattern reported in attenuated B. bovis. Clonal cell lines were derived by limited dilution from this "up-regulated sbp2t11 mixed population" cell line. Characterization of the clonal lines showed significantly lower cytoadhesion to bovine brain endothelial cells compared to the wildtype virulent strain. Cytoadhesion selection demonstrated a increment in cyto-adhesion among virulent strain and CD7 control which correlated with down-regulation of sbp2t11; clones 11D, 12A and 12B did not increased cytoadhesion which correlate with up-regulation of sbp2t11; clones 3A and 3B increased cytoadhesion which correlated with a switch in transcript level of sbp2t11; it was not possible to selected attenuated strain by cyto-adhesion phenotype. Thus, we demonstrated an association between up-regulation of sbp2t11 and low cytoadhesion.