HSPA4 and neurodegenerative disease: Identification of the divergent functions for different Hsp70 variants under stress, together with the threshold effects defined for Hsp70 and J‐protein efficacy during massive protein aggregation, suggests specific molecular ratios of protein quality control elements inside a cell (Manzerra et al., 1997; Chen & Brown, 2007) may be central to degenerative disease development.