Fluid biomarkers for amyotrophic lateral sclerosis (ALS): a review Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the loss of upper and lower motor neurons. Currently, three FDA-approved drugs are available to slow functional decline in ALS patients, but no cure exists. With an average life expectancy of two to five years after diagnosis, there is a clear need for biomarkers to improve ALS care and expedite treatment development. This review summarizes efforts to identify diagnostic, prognostic, susceptibility/risk, and response fluid biomarkers for ALS, aiming to facilitate earlier diagnosis, better prognosis prediction, improved clinical trial design, and interpretation of trial results. Over 20 years, several promising fluid biomarker candidates for ALS have emerged, including neurofilaments, TDP-43, and dipeptide repeat proteins. Neurofilaments, particularly neurofilament light (NfL) and phosphorylated neurofilament heavy (pNfH), are widely studied as fluid biomarkers for ALS. Elevated NfL and pNfH levels in CSF, plasma, and serum are associated with ALS and other neurological diseases. These biomarkers show promise as diagnostic, prognostic, susceptibility/risk, and response markers. Higher NfL levels in CSF correlate with shorter survival and more severe disease. Neurofilaments may also predict disease progression and help stratify patients in clinical trials. TDP-43 is a key protein in ALS pathology, with cytoplasmic inclusions being the main pathological hallmark. TDP-43 levels in CSF and plasma are being studied as potential biomarkers. While TDP-43 levels in CSF show some utility as an early marker of disease, their association with survival and functional outcomes is complex. TDP-43-related biomarkers, including phosphorylated TDP-43 and autoantibodies, are being explored for their diagnostic and prognostic potential. Dipeptide repeat proteins (DPRs), particularly poly(GP), poly(GA), and poly(GR), are also being studied as pharmacodynamic biomarkers. These proteins are associated with C9orf72-related ALS and FTD. DPR levels in CSF and plasma are being evaluated for their potential as biomarkers for therapeutic approaches targeting C9orf72 repeat RNA. Cryptic exons, resulting from TDP-43 dysfunction, are being investigated as potential biomarkers. These exons are incorporated into RNA during splicing and may serve as early diagnostic markers for ALS. Cryptic exons in genes like HDGFL2 are being studied for their potential as biomarkers in CSF. Overall, fluid biomarkers such as neurofilaments, TDP-43, and DPRs show promise in improving ALS diagnosis, prognosis, and treatment development. However, further research is needed to validate their utility and refine their application in clinicalFluid biomarkers for amyotrophic lateral sclerosis (ALS): a review Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the loss of upper and lower motor neurons. Currently, three FDA-approved drugs are available to slow functional decline in ALS patients, but no cure exists. With an average life expectancy of two to five years after diagnosis, there is a clear need for biomarkers to improve ALS care and expedite treatment development. This review summarizes efforts to identify diagnostic, prognostic, susceptibility/risk, and response fluid biomarkers for ALS, aiming to facilitate earlier diagnosis, better prognosis prediction, improved clinical trial design, and interpretation of trial results. Over 20 years, several promising fluid biomarker candidates for ALS have emerged, including neurofilaments, TDP-43, and dipeptide repeat proteins. Neurofilaments, particularly neurofilament light (NfL) and phosphorylated neurofilament heavy (pNfH), are widely studied as fluid biomarkers for ALS. Elevated NfL and pNfH levels in CSF, plasma, and serum are associated with ALS and other neurological diseases. These biomarkers show promise as diagnostic, prognostic, susceptibility/risk, and response markers. Higher NfL levels in CSF correlate with shorter survival and more severe disease. Neurofilaments may also predict disease progression and help stratify patients in clinical trials. TDP-43 is a key protein in ALS pathology, with cytoplasmic inclusions being the main pathological hallmark. TDP-43 levels in CSF and plasma are being studied as potential biomarkers. While TDP-43 levels in CSF show some utility as an early marker of disease, their association with survival and functional outcomes is complex. TDP-43-related biomarkers, including phosphorylated TDP-43 and autoantibodies, are being explored for their diagnostic and prognostic potential. Dipeptide repeat proteins (DPRs), particularly poly(GP), poly(GA), and poly(GR), are also being studied as pharmacodynamic biomarkers. These proteins are associated with C9orf72-related ALS and FTD. DPR levels in CSF and plasma are being evaluated for their potential as biomarkers for therapeutic approaches targeting C9orf72 repeat RNA. Cryptic exons, resulting from TDP-43 dysfunction, are being investigated as potential biomarkers. These exons are incorporated into RNA during splicing and may serve as early diagnostic markers for ALS. Cryptic exons in genes like HDGFL2 are being studied for their potential as biomarkers in CSF. Overall, fluid biomarkers such as neurofilaments, TDP-43, and DPRs show promise in improving ALS diagnosis, prognosis, and treatment development. However, further research is needed to validate their utility and refine their application in clinical