Fig. 7: Proposed model of the activation of the Csn1s2a/b locus during pregnancy and lactation.

The Csn1s2b gene is under the control of three distinct enhancers; a distal enhancer controlling gene expression throughout lactation, an intronic enhancer active at late pregnancy and early lactation, and a super-enhancer (SE) required for the activation of the Csn1s2b locus. While the SE has limited influence on the expression of the Csn1s2a and Odam genes, it does not activate the Prr27 gene in mammary tissue. Two candidate enhancers have been identified in the Csn1s2a distal region, but their function has not been validated through experimental genetics. The color intensity of promoter and enhancer elements at late pregnancy and peak lactation reflects their relative strengths. Darker colors indicate increased strength. Candidate Csn1s2a enhancer and promoter elements are shown but their biological strength is not known. CTCF binding sites have been identified³⁸.