CCL2 and amyotrophic lateral sclerosis: Interestingly, the impact of heterogeneity was reduced to zero (Q = 2.53, d.f. = 5, I2 = 0, P = 0.772), and the significant association between CSF MCP-1 and ALS was slightly increased for the ES generated by sample size and P-value group (6 studies, Hedges' g = 0.685, 95% CI = 0.479 to 0.891, P < 0.001).