The full-length nature of the released CK19 protein was confirmed by proteomic experiments on MCF-7 breast cancer cell supernatants: (a) we did not detect a specific tryptic fragment peptide, which should have been detected if the CYFRA 21-1 were present in the culture supernatant; and (b) we found a specific hydrophobic peptide (amino acids 227 to 247), which can emerge only from the intact insoluble CK19 protein. This evidence concerns the gene KRT19 and breast cancer.