The presence of an NR box with the LXXLL motif into Api5 sequence was a good clue as we were able to demonstrate that: (i) Api5 and ERα colocalized in breast carcinoma cells, (ii) Api5 and ERα belonged to the same complex, (iii) Api5 LXXLL motif was necessary for complex formation with ERα, (iv) Api5 interacted directly through ERα C domain. The gene discussed is API5; the disease is breast carcinoma.