Abstract: A process for isolating or extracting rare cells from a biological sample comprising filtering a biological sample, which may be treated or diluted, through a filter that has a pore size, pore density or other physical properties that retain rare cells, but which permits other kinds of cells to pass through the filter. This process also comprises multiple analyses performed on rare cells after their extraction or isolation by filtration to diagnostically identify the presence of rare cells in a biological sample and to use their diagnostic identification and molecular characterization for diagnostic purposes such as for early diagnosis of diseases, namely for early diagnosis of cancer and to select, guide, monitor treatments and in particular to select targeted treatments and to monitor the response and/or resistance to them.