PFN1 and amyotrophic lateral sclerosis: However, the evidence supporting a gain-of-function mechanism is far more robust, since the transgenic rodents expressing mutant (C71G, G118V) PFN1 forms elicit motor neuron degeneration and ALS-like phenotypes [22,23,24,25], whereas heterozygous Pfn1 knockout mice do not show overt motor phenotypes [26].