and genome-wide association studies identified dachshund homolog 1 (DACH1), which suppresses tubular cell cycle progression, as having protective effects on CKD (7). Though suggestive that ongoing tubular cell cycling may be detrimental, few studies have directly manipulated cell cycle progression in the chronically injured kidney to determine how tubular cell cycle alters tubular injury and fibrosis.Most proximal tubules in the uninjured kidney are quiescent (G0) but, upon injury, reenter the cell cycle at G1, which is the first cell cycle checkpoint. Actively cycling cells progress from G1/S phase (DNA synthesis); then to the second checkpoint, G2; and finally to M (mitosis). Work has established that some tubule cells arrest at G2/M following injury, and these G2-arrested cells play an important role in CKD progression through production of profibrotic growth factors like TGF-β and CTGF (8). However, little is known about the role of cell cycle progression at the G1/S transition in chronically injured tubules. Cyclin dependent kinases 4 and 6 (CDK4/6) combine with cyclin D1 to phosphorylate retinoblastoma protein (Rb), leading to the liberation of E2F, a transcription factor that promotes expression of genes related to G1/S phase progression. The FDA-approved drug palbociclib is a highly selective inhibitor of CDK4/6 that is currently used to treat patients with breast cancer and provides an important tool to assess how G1/S cell cycle progression alters CKD progression.In this manuscript, we use a cell cycle reporter mouse to confirm that cell cycle is persistently altered in the chronically injured proximal tubules. Blocking CDK4/6 in mice after 2 different models of CKD significantly reduced tubular cell cycle progression, ameliorated tubular injury, preserved renal function, and decreased tubulointerstitial fibrosis. Surprisingly, selective tubular deletion of cyclin D1 worsened, rather than improved, tubular injury in murine CKD. Human expression quantitative trait loci (eQTLs) and genome-wide association study (GWAS) data are consistent with the murine studies showing that inhibiting CDK4/6, but not cyclin D1, protects against CKD. Mechanistically, CDK4/6 inhibition reduced proximal tubule cell death through a pathway involving STAT3/IL-1β.