Bovine milk casein (CN) account for about 80% of the total proteins in milk. Genes encoding bovine CN are in chromosome 6. The most common alleles in dairy cattle are A1 and A2, being the former one a genetic variation of A2 that happened thousands of years ago and affected European cattle origins. Variants A1 and A2 apparently occurs at the same allele frequencies in Holstein cows. Dairy milk protein profile can be influenced by many factors as breed, lactation stage, mastitis, and diet composition. The way these variants affect milk protein composition is off special interest due to their effect on dairy products processability and functionality, and their effect on human health. A database from 13 experiments completed at the University of Illinois (Urbana-Champaign) from 2016 to 2018 was developed. A total of 142 cows (117 multiparous and 25 primiparous) was included in the analyses. Cows β-CN genetic evaluation (i.e.; A1_A1, A1_A2; and A2_A2) for 128 cows was performed (Clarifide. Zoetis, Kalamazoo, MI). Treatments were as follow: cows A1_A1, cows A2_A2, and cows A1_A2. Parity was dichotomized as cows starting first lactation in one group (LAG1), cows starting second or third lactation in a second group (LAG2), and cows in the fourth-or-greater lactation in a fourth group (LAG3). Data were analyzed using the MIXED procedure of SAS, using 2 orthogonal contrasts. Contrast 1 (CONT1): A1_A1 compared with A2_A2 and contrast 2 (CONT2): A1_A1 compared with the average of A2_A2 and A1_A2. Milk yield was greater for cows A1_A1 than cows A2_A2 and A1_A2 (35.63 vs 34.24 ± 0.63 kg/d; P = 0.03, CONT2). There were no milk protein yield differences among treatments (P > 0.1, CONT1 and CONT2). There were no milk casein as a percentage of protein differences among treatments (P > 0.1, CONT1 and CONT2). Milk lactose yield was greater for cows A1_A1 than cows A2_A2 and A1_A2 (1.69 vs 1.61 ± 0.04 kg/d; P = 0.02, CONT2). Cows A1_A1 tended to have greater milk lactose yield than cows A2_A2 (1.69 vs 1.62 ± 0.04 kg/d; P = 0.06, CONT1). In conclusion, cows homozygotes A1_A1 had similar milk protein yield and milk casein yield than homozygotes A2_A2 and heterozygotes A1_A2. Cow homozygotes A1_A1 had the greatest milk yield and lactose yield.