In addition, both M-type CTC counts and the information from the analysis of multidrug resistance-associated protein 2 (MRP2) and MRP5 gene expression of the CTCs harvested via the 2-step CTC isolation protocol were able to serve as prognostic factors of progression-free survival (PFS) in head and neck squamous cell carcinoma (HNSCC). This evidence concerns the gene ABCC5 and head and neck squamous cell carcinoma.