Caseins constitute the main part of milk proteins. The genes for the four major types of casein (CN), αs1-CN, αs2-CN, β-CN and κ-CN, reside on < 220 kb of the DNA on bovine chromosome 6 (Threadgill & Womack, 1990). Two casein genes (Cn), β-Cn and κ-Cn, have been found to be associated with differences in milk production, cheese yield, protein content, quality and fat yield (Ng-Kwai-Hang et al. 1984). Lin et al. (1986) have estimated that the combined contributions of αs1-Cn, β-Cn, κ-Cn and β-lactoglobulin loci accounted for 8·9% of the total phenotypic variance in milk, 8·6% of that in proteins and 5% of that in fat yield. Six alleles of κ-Cn gene have so far been identified: κ-CnA, κ-CnB, κ-CnC, κ-CnE, κ-CnF and κ-CnG. The κ-CN protein is 169 amino acids long with varying regions at codons 136 and 148. The A variant has threonine (ACC) at codon 136 and aspartic acid (GAT) at codon 148, whereas the B variant has isoleucine (ATC) and alanine (GCT) at codons 136 and 148 respectively (Mercier et al. 1973). Milk containing κ-CN B was reported to yield more cheese than that with κ-CN A because it contains more casein and has lower fat loss. Several studies have shown that cows with the κ-CnBB genotype produced milk with a higher protein content than cows with κ-CnAA (Ng-Kwai-Hang et al. 1984; Ron et al. 1994).