Second, despite the ability to detect rare cell populations (as few as 0.01% of cells in the human lung cell atlas), single-cell RNA sequencing studies have not detected a population of transcriptionally distinct cells simultaneously expressing CDKN2A and SASP genes in the normal human lung (150), or in diseases like pulmonary fibrosis, where senescence has been suggested to play a role (15–17). This evidence concerns the gene CDKN2A and pulmonary fibrosis.