What happened: Former NFL running back Chris Johnson announced he has been diagnosed with ALS, the neurodegenerative disorder known as Lou Gehrig's disease. Johnson, 39, made the diagnosis public this week. The three-time Pro Bowl back was one of the most explosive runners of his era during a decade-long career.

Why it matters: Johnson is among the most accomplished rushers of his generation, and an ALS diagnosis at 39 lands hard across the football community. The disease is progressive and currently has no cure, making the news a significant moment for fans and former teammates who followed his career. The announcement drew an immediate wave of support online.

What to watch: Watch for further statements from Johnson on his treatment and next steps, along with reaction from the broader NFL community.