The aims of this observational prospective study were twofold: (a) to compare the performance of the intraoperative OSNA assay with conventional postoperative histological procedures in 903 SLNs sampled from 709 early BC patients (b) to determine whether the CK19 mRNA copy number in the SLN may predict the risk of a positive ALND within the different BC molecular subtypes. The gene discussed is KRT19; the disease is breast cancer.