Add reference In the present study, we used a functional proteomic approach to identify Annexin A1 (Anxa1) interacting proteins in the Philadelphia-positive KCL22 cell line. We focused on Anxa1 because it is one of the major proteins upregulated in imatinib-sensitive KCL22S cells versus imatinib-resistant KCL22R. Our proteomic strategy revealed 21 interactors. Bioinformatic analysis showed that most of these proteins are involved in cell death processes. Among the proteins identified, we studied the interaction of Anxa1 with two phosphatases, Shp1 and Shp2, which were recently identified as biomarkers of imatinib sensitivity in patients affected by chronic myeloid leukemia. Our data open new perspectives in the search for annexin-mediated signaling pathways and may shed light on mechanisms of resistance to imatinib that are unrelated to Bcr-Abl activity. For data analysis, we used the MASCOT software (version 2.4) Peptide Mass Fingerprinting search program (http://www.matrixscience.com) selecting NCBInr aug2010 database (11592891 sequences) (http://www.ncbi. nlm.nih.gov), and the following six parameters: specificity of the proteolytic enzyme used for hydrolysis (trypsin); up to 1 missed cleavage; cysteines such as S-carbamidomethylcysteines; unmodified N- and C-terminal ends; unmodified and oxidized methionines; putative Gln-induced pyroGlu formation; a precursor peptide maximum mass tolerance of 400 ppm, and a maximum fragment mass tolerance of 0.6 Da. Using the probability-based Mowse score, the ion score is 10 x Log(p), p being the probability that an eventual match is a random occurrence. All MS/MS spectra with a MASCOT score of or higher than 41 (p =< 0.05) had a good S/N, so the interpretation of the data was unambiguous. MS/MS spectra of peptides that had a MASCOT score below 41 were inspected manually and included in the statistics only in the presence of at least four continuous y or b ions.