Caseins (α_s1, β, α_s2, e κ) represent about 80% of the whole protein content of ruminant milk. Each of these proteins is encoded by single copy genes (CSN1S1, CSN2, CSN1S2 and CSN3, respectively) clustered on a ∼200-kb segment of chromosome 6 (Ferretti et al. 1990; Gallagher et al. 1994) in the order: CSN1S1, CSN2, CSN1S2 and CSN3 (Mercier & Vilotte, 1993). Furthermore, in cattle and goat CSN1S1 and CSN2 are convergently transcribed (Leroux & Martin, 1996; Rijnkles et al. 1997) and are only 20 and 12 kb apart, respectively.