HDGFL2 and amyotrophic lateral sclerosis: A clearer understanding of the temporal course of cryptic HDGFL2 in pre-symptomatic disease and its potential use as a susceptibility/risk biomarker predicting phenoconversion to ALS or FTD will require more sensitive cryptic HDGFL2 assays and longitudinal measures of cryptic HDGFL2 in samples obtained pre- and post-conversion.