Although pregnancy-related factors such as nulliparity and late age at first full-term pregnancy are well-established risk factors for invasive breast cancer, it remains unclear whether such factors have similar effects on breast carcinoma in situ (CIS) or whether their effects vary across the subtypes of invasive breast cancer defined by the estrogen receptor (ER) or the joint ER and progesterone receptor (PR) status of the tumor. Here, ESR1 is linked to in situ carcinoma.