肺换气发生在结构精妙的肺泡囊中，其内壁有两种上皮细胞类型：鳞状1-型肺泡细胞(AT1细胞)和骰状2-型肺泡细胞(AT2细胞)。前者介导换气；后者分泌防止肺泡在呼吸过程中崩溃的表面活性剂。Mark Krasnow及同事采用肺泡标记物、遗传世系追踪和克隆分析来在小鼠整个生命周期中对肺泡祖细胞进行活体识别。他们发现，AT1和AT2细胞是在发育过程中从一种“双潜力”祖细胞形成的。在出生后，成熟AT2细胞起“兼性”干细胞的作用，形成再生肺泡的慢慢增大的单克隆位点。有致癌作用的Kras (G12D) 突变永久性地激发AT2自我更新，劫持这种“兼性”干细胞功能来启动肺癌。
Alveoli are gas-exchange sacs lined by squamous alveolar type (AT) 1 cells and cuboidal, surfactant-secreting AT2 cells. Classical studies suggested that AT1 arise from AT2 cells, but recent studies propose other sources. Here we use molecular markers, lineage tracing and clonal analysis to map alveolar progenitors throughout the mouse lifespan. We show that, during development, AT1 and AT2 cells arise directly from a bipotent progenitor, whereas after birth new AT1 cells derive from rare, self-renewing, long-lived, mature AT2 cells that produce slowly expanding clonal foci of alveolar renewal. This stem-cell function is broadly activated by AT1 injury, and AT2 self-renewal is selectively induced by EGFR (epidermal growth factor receptor) ligands in vitro and oncogenic Kras(G12D) in vivo, efficiently generating multifocal, clonal adenomas. Thus, there is a switch after birth, when AT2 cells function as stem cells that contribute to alveolar renewal, repair and cancer. We propose that local signals regulate AT2 stem-cell activity: a signal transduced by EGFR-KRAS controls self-renewal and is hijacked during oncogenesis, whereas another signal controls reprogramming to AT1 fate.