Microarray analysis was used to identify genes whose expression in the mammary gland of Holstein-Friesian dairy cows was affected by the nonconservative Ala to Lys amino acid substitution at position 232 in exon VIII of the diacylglycerol-O-transferase 1 (DGAT1) gene. Mammary gland biopsies of 9 homozygous Ala cows, 13 heterozygous cows (Ala/Lys), and 4 homozygous Lys cows in midlactation were taken. Microarray ANOVA and factor analysis for multiple testing methods were used as statistical methods to associate the expression level of the genes present on Affymetrix bovine genome arrays (Affymetrix Inc., Santa Clara, CA) with the DGAT1 gene polymorphism. The data was also analyzed at the level of functional modules by gene set enrichment analysis. In this small-scale experimental setting, DGAT1 gene polymorphism did not modify milk yield and composition significantly, although expected changes occurred in the yields of C14:0, cis-9 C16:1, and long-chain fatty acids. Diacylglycerol-O-transferase 1 gene polymorphism affected the expression of 30 annotated genes related to cell growth, proliferation, and development, remodeling of the tissue, cell signaling and immune system response. Furthermore, the main affected functional modules were related to energy metabolism (lipid biosynthesis, oxidative phosphorylation, electron transport chain, citrate cycle, and propanoate metabolism), protein degradation (proteosome-ubiquitin pathways), and the immune system. We hypothesize that the observed differences in transcriptional activity reflect counter mechanisms of mammary gland tissue to respond to changes in milk fatty acid concentration or composition, or both.