PGR and cancer: For instance, increased cysteine availability in plasma is related to an enhanced risk of breast cancer, particularly oestrogen receptor-positive (ER+) and/or progesterone receptor-positive (PR+) cancer and in combination with low folate availability in plasma.182 In breast cancer, higher cysteine plasma concentrations are found in patients with higher body weight,182,183 which is a known risk factor for breast cancer, progression and mortality,184 further linking cysteine, metabolic dysregulation and cancer.