Recent studies have revealed that, in addition to hormones and other protein 
factors, noncoding RNA molecules play an important regulatory role in milk 
protein synthesis. Circular RNAs (circRNAs) are universally expressed noncoding 
RNA species that have been proposed recently to regulate the expression of their 
parental genes. In the present study, the deep RNA-sequencing technique known as 
RNA-seq was used to compare expression profiles of circRNAs from 2 pooled RNA 
samples from cow mammary gland on d 90 and 250 postpartum and to identify the 
key circRNAs involved in lactation. A total of 4,804 and 4,048 circRNAs were 
identified in the cow mammary gland on d 90 and 250 postpartum, respectively, of 
which only 2,231 circRNAs were co-expressed at both lactation stages, suggesting 
high stage specificity in the circRNAs. The enrichment of some Gene Ontology 
terms for the circRNA parental genes differed between lactation stages. Among 
the top 10 enriched Gene Ontology terms, vesicle, endoplasmic reticulum, and 
mitochondrial lumen were more common on lactation d 90. All 4 casein-coding 
genes (CSN1S1, CSN1S2, CSN2, and CSN3) produced circRNAs in the cattle mammary 
gland. In total, 80 circRNAs were identified from these 4 genes; circRNAs from 
CSN1S1 had very high abundance, and 3 of them accounted for 36% of all the 
circRNAs expressed in the mammary gland on lactation d 90. Three circRNAs from 
CSN1S1, 1 circRNA from CSN1S2, and 1 circRNA from CSN2 were all more highly 
expressed on lactation d 90 than on lactation d 250, as confirmed by 
quantitative PCR. These circRNAs had several target sites for the microRNA 
miR-2284 family and were predicted to target CSN1S1 and CSN2 mRNA, suggesting 
their potential involvement in regulating expression of the casein genes.