What happened: Chris Johnson, a former NFL running back, has revealed that he has been diagnosed with ALS at age 39. He shared the diagnosis publicly in an interview.

Why it matters: ALS, or amyotrophic lateral sclerosis, is a progressive and incurable neurodegenerative disease, making this a sobering development for a player who built a standout career in the league. The disclosure adds Johnson to the list of football figures who have confronted serious neurological illness, a topic that continues to weigh on the sport and its alumni.

What to watch: Watch for an outpouring of support from across the NFL community and for any further word from Johnson on his treatment and advocacy plans.