SOD1 and amyotrophic lateral sclerosis: To determine to which extent increased surface expression of P2X4 contributes to ALS onset and progression, we crossed SOD1 transgenic mice (SOD1G93A) with constitutive internalization-defective P2X4mCherryIN knockin mice (P2X4KI) to obtain SOD1:P2X4KI (and littermate WT:P2X4KI) mice expressing enhanced surface levels of P2X4 [48].