FAP and pancreatic ductal adenocarcinoma: Consistent with the observation that CTCs are heterogeneous in nature and that a specific CTC population isolated by a given method is a predictor for therapeutic response, SMAD4 expression of a CTC population called circulating tumor and invasive cells (CTICs) was able to predict response of pancreatic ductal adenocarcinoma.9 In addition, CTCs expressing fibroblast activation protein alpha (FAPα)/seprase and epithelial cell adhesion molecule are distinct subpopulations of CTCs and the use of these markers in concert could provide information concerning therapeutic response.10