In this study, the effect of different clones bearing dominant‐negative alleles of hns such as hnsP116S, hnsT55P, hnsL26P, hns∆64 and hns+, in phenotypically Ces strains such as MMRT6 (∆lon rpoB12) and MMRT23 (∆lon rpoB77) and also in ∆lon and lon+ strains has clearly revealed that the hnsP116S allele that codes for H‐NS but is defective in recognizing curved DNA almost completely abolished the elicitation of Ces phenotype in both ∆lon rpoB12 and ∆lon rpoB77 strains. The gene discussed is LONP1; the disease is cat-eye syndrome.