The polymorphisms of exon 4 and 5 of κ-casein (CSN3) gene and their associations with milk production traits and expression pattern in Chinese Holstein cattle were investigated. Nine mutational sites, of which seven were novel mutational sites, were identified and genotyped by polymerase chain reactionrestriction fragment length polymorphism (PCR-RFLP), created restriction site-PCR (CRS-RFLP) and sequencing methods in 398 cows. Linkage disequilibrium analysis showed that SNP-1 (g.10891 T > C rs 43703015, g.10927 C > A rs 43703016, g.10988 G > A ss 256302464 and g.10966 A > T ss 256302465) and SNP-2 (g.12907 A > G ss 256302466, g.12950 G > A ss 256302468, g.12989 C > T ss 256302469 and g.13028 A > G ss 256302470) were completely linked, respectively. Correlation analysis showed that SNP-1, SNP-2 and SNP-3 (g.12980 T > C ss 256302467) markers were closely correlated to the fat content. The SNP-3 marker had a remarkable effect on the protein content (P < 0.05). 16 combined genotypes of the three SNPs were found. Fat and protein content in combinations of genotypes were varied significantly (P < 0.05). Genotypes BBCCEE and ABTCDD individuals had the highest fat and protein content, respectively, which may be useful for marker assisted selection program in dairy cattle. The expression of CSN3 mRNA in the mammary tissue was higher than that of in the liver tissue (P < 0.05) and the expression in the spleen of BB genotype was higher than that of AA genotype in the SNP-1 (P < 0.05) by fluorescent quantitation real-time PCR (Q-PCR) assay.Key words: SNPs, CSN3 gene, combined genotype, Q-PCR, milk production traits
