Abstract:Because of the antimicrobial role that defensins play in cows, genes encoding these peptides may be considered as molecular markers of a genetically determined susceptibility of the mammary gland to mastitis. In Friesian and Egyptian cows which were selected based on their milk productivity, 1638-bp beta-defensin genes were amplified. Two PCR amplicon sizes of the gene encoding beta-defensin (1638 bp and 429 bp) were observed in Friesian cows (high milk production), while in Egyptian cows (low milk production) one PCR amplicon size (429 bp) was observed. PCR-RFLP technique was used to discriminate between the common 429 bp band in both Friesian and Egyptian cows, but no difference between them had been observed. DNA sequencing for 1638 bp (B2) and 429 bp (B1) was carried out. Sequence analysis indicated that these two PCR amplicon sizes were two types of genes encoding beta-defensins and very tightly close to each other. Based on their sequence alignment (B1 and B2) with the presented defensin genes in the GenBank, phylogenic tree was constructed. A new gene (B1) belongs to the beta-defensin genes family was detected for the first time and associated to the low milk production in cattle population.