SPP1 and amyotrophic lateral sclerosis: Several well studied serum and CSF biomarkers of ALS progression were also present, such as SPP1 [61], the human chitinases CHI3L1 and CHI3L2 [62, 63], and complement C3 [64], in addition to prognostic and predictive CSF biomarkers such as TREM2, LILRA2 and ITGB2 [65].