HOXD13 and amyotrophic lateral sclerosis: Similarly, elevated levels of secreted PD-1 (sPD-1) and secreted PD-L1 (sPD-L1) were also observed in the serum of patients with ALS and relapsing-remitting multiple sclerosis (RRMS) (Beers et al., 2021; Tsaktanis et al., 2023).