To further characterize the mechanisms leading to the transient increase in F-actin and cell stiffness, we searched for ABPs involved in building these F-actin structures. Microarray profiling identified 35 ABPs deregulated during the 36 h of Src-induced cellular transformation (Fig. 3a)18. To test which of these ABPs would be relevant for breast cancer progression, we analysed the expression signature related to the actin cytoskeleton in premalignant atypical ductal hyperplasia (ADH), ductal carcinoma in situ (DCIS) and invasive ductal carcinoma (IDC) (Fig. 3b). Using cancer expression data sets available from the GEO database, we collected microarray data for 255 normal breast tissues and 903 neoplastic breast lesions, from which the available information relative to the diagnosis, histological grade and ER status was extracted (Supplementary Table 1). After normalization of the raw data and statistical comparison of breast lesions with normal breast tissues, we sorted all genes significantly deregulated (Supplementary Data 1). As expected, classification of genes into functional categories using Pathway Express indicated that most of the significantly affected biological processes were associated with cancer (Fig. 3b). Strikingly, ‘regulation of the actin cytoskeleton’ was among the top 10 pathways significantly deregulated in both premalignant lesions (ADH and DCIS). In contrast, while IDCs upregulated many ABPs previously implicated in cancer cell mobility23 (Supplementary Data 2), the category ‘regulation of the actin cytoskeleton’ had a lower impact factor in these malignant lesions relative to other misregulated pathways or was not being significantly affected (Fig. 3b). In addition, classification of these ABPs into functional categories based on their role in F-actin dynamics (Supplementary Data 2) showed that only malignant lesions were enriched for inhibitors of polymerization (Supplementary Fig. 4). Thus, major alterations in the expression of ABPs are predominantly found in premalignant ADH and DCIS.