CCL2 and susceptibility to pneumonia measurement: Our finding that pro-inflammatory gene expression is stronger in BALB/c mice is in line with several previous studies that compared C67Bl/6 and BALB/c mice: After infection with mcyoplasma pneumonia, the BAL levels of CXCL1, MIP-1α, MCP-1, IL-1b and IL-6 were higher in BALB/c mice [40].