The unprecedented potential of single cell analysis techniques to detect rare cell populations will undoubtedly lead to discovery of CD4 CTL in other pathologies, as was recently demonstrated for COVID-19 (78), HIV (79), Parkinson’s disease (80), Sjögren’s disease (81), colitis (82), colorectal cancer (83), and cutaneous T cell lymphoma (84). This evidence concerns the gene CD4 and Parkinson disease.