The lack of expression of ER-α and/or PR in the majority of rabbit mammary carcinomas does not completely rule out an influence of estrogen and progesterone on tumorigenesis, since at least one of these receptors is expressed in 68% of the mammary cysts (n = 17) and all examined cases of lobular hyperplasia (n = 2), adenoma (n = 3) and in situ carcinoma (n = 2) [11]. This evidence concerns the gene PGR and cancer.