Expression of NKX3.1 and P501S was identified in 8 (32%) and 7 (28%) cases of invasive breast carcinomas, 42% and 20% of in situ breast carcinomas, and 8% and 46% of normal terminal ductal lobular unit (TDLU), respectively. The gene discussed is NKX3-1; the disease is lobular breast carcinoma in situ.