PGR and breast cancer: The mRNA and protein expressions of the c-myc, cyclin D1, and PgR genes are co-regulated by ERα and AIB1 (Fig. 8C and D); therefore, the E2-ERα/AIB1-c-myc, cyclin D1, and PgR pathways are likely to be involved in the preferential growth of ERα-positive breast cancer cells on hard substrates.