Abnormal　expression　of　CBP/p300-interacting　transactivators　with　glutamic　acid　[E]　and　aspartic　acid　[D]-rich　C-terminal　domain1　(Cited1)　has　been　reported　in　several　human　cancers.　However,　little　information　is　available　regarding　the　effects　of　Cited1　on　non-small-cell　lung　cancer　(NSCLC).　The　present　study　was　aimed　to　explore　the　effects　of　downregulation　of　Cited1　on　NSCLC,　as　well　as　the　underlying　mechanism.　NSCLC　cell　lines　A549,　HCC827　and　H1975　cells　were　transfected　with　small　interfering　RNA　(siRNA)　against　Cited1　(siCited1).　After　transfection,　the　expression　of　Cited1　was　confirmed　by　quantitative　RT-PCR　(qRT-PCR)　and　Western　blot.　Cell　viability　was　analyzed　by　3-(4,　5-dimethylthiazol-2-yl)-2,　5-diphenyltetrazolium　bromide　(MTT)　assay　and　cell　cycle　analysis　was　determined　by　flow　cytometry.　In　addition,　expression　of　cyclin-dependent　kinase　1　(CDK1),　phosphorylated　CDK1　(p-CDK1),　cell　division　cycle　25C　(CDC25),　and　Cyclin　B1　was　measured　by　qRT-PCR　and　Western　blot.　Both　the　mRNA　and　protein　levels　of　Cited1　were　significantly　down-regulated　by　transfection　with　siCited1　(P　＜　0.05).　Transfection　with　siCited1　significantly　decreased　the　cell　viability　(P　＜　0.05)　and　the　percentages　of　cells　in　G1　phase,　while　markedly　increased　the　percentages　of　cells　in　G2/M　phase　in　the　three　cell　lines.　Both　the　mRNA　and　protein　levels　of　p-CDK1,　CDK1,　CDC25C　and　Cyclin　B1　were　statistically　reduced　by　downregulation　of　Cited1　compared　to　the　control　group　(P　＜　0.05).　However,　no　significant　differences　were　observed　in　CDK1.　Downregulation　of　Cited1　suppresses　cell　proliferation　by　inducing　G2/M　arrest　of　the　cell　cycle　in　NSCLC　cell　lines.