RB1 and breast cancer: Gas6/TAM have critical roles in other cancers, such as breast cancer and melanoma.40, 56, 65, 98, 99, 100, 101 Gas6 is overexpressed and amplified in breast cancer and can be upregulated by progesterone via progesterone receptor B (PRB).99 By using quantitative real-time PCR analysis of the levels of Gas6 mRNA expression in 49 primary breast carcinomas and evaluating the expression of the PRB protein immunohistochemically with a commercially available PRB antibody, one study showed a positive association between the PRB protein and Gas6 mRNA levels.