The protein with the largest effect size was FGF-21, (β = 0.68; 95% CI, 0.39-0.97; Q =.002), and tPA presented the second-largest effect size (β = 0.21; 95% CI, 0.13-0.29; Q <.001), both with a faster rise from PNA day 0 to 4 in severe ROP cases (eTable 3 in Supplement 4). Here, FGF21 is linked to retinopathy of prematurity.