Expression of ID2 in NSCLC Figure 1. ID2 immunohistochemistry in normal bronchial epithelium (A–B) and in NSCLC (C–H). A–B: ID2 staining of epithelial and endothelial cells in non-cancerous lung. C–F: Representative expression on TMA with strong ID2 staining in nucleus and with moderate (C, D) or weak (E, F) cytoplasmic staining. C, D are poorly differentiated adenocarcinomas and E, F is a poorly differentiated large cell carcinoma. G–H: Tumor with absence of nuclear staining with moderate (G, H) cytoplasmic expression of ID2. G, H are moderately differentiated squamous cell carcinomas. doi:10.1371/journal.pone.0004158.g001 demonstrated that high ID2 expression in nuclei seems to be correlated with poor differentiation of the tumor. This observation is well in agreement with known properties of ID2, since the protein has been characterized as an inhibitor of differentiation in numerous tissues, including the epidermis and muscle [18,19]. Thus, ID2 inhibits muscular differentiation by direct interaction with MyoD, a bHLH transcription factor required for myoblastic differentiation [20]. Moreover, in prostate carcinoma, strong ID2 expression was associated with poorly differentiated grades of tumor, suggesting that ID2 could also be involved in regulation of tumor dedifferentiation [11,21]. Our data showed that a high nuclear expression level of ID2 was an independent and negative prognostic marker of survival in patients with poorly differentiated lung tumors. Thus, patients with tumors exhibiting high nuclear expression of ID2 have 13.7-fold higher risk of death than patients with low expression of ID2. The prognostic value of high ID2 expression has already been demonstrated in other cancer types such as neuroblastoma and prostate cancer [7,11,21,22,23]. However, previous studies on breast cancer and SCLC have reported that high cytoplasmic expression of ID2 was associated with a good prognosis, whereas no correlation was found between nuclear expression of ID2 and the survival of patients [13,22]. Kamalian et al. suggested that in SCLC increased expression of ID2 in cytoplasm could be due to its export from nuclei and may PLoS ONE | www.plosone.org 3 January 2009 | Volume 4 | Issue 1 | e4158