The severity of Mycoplasma-like lesions and the prevalence of pneumonia were higher in batches of CAT 2 than in batches of CAT 1 and significantly higher in batches of CAT 3 than in batches of CAT 1 (P = 0.064 for CAT 2 to CAT and P = 0.012 for CAT 3 to CAT 1 and OR: 1.30; P = 0.33 for CAT 2 to CAT 1 and OR: 2.08; P = 0.012 for CAT 3 to CAT 1, respectively for the severity of Mycoplasma-like lesions and the prevalence of pneumonia). The gene discussed is CAT; the disease is pneumonia.