Since calmodulin (CaM), a ubiquitous cellular calcium sensor, is often overexpressed in breast cancers, especially in estrogen receptor-positive breast tumors and enhances the stability of estrogen receptor [20, 21], we examined whether it also protects TBC1D3 from GF-induced degradation in two distinct cell culture models of human breast cancer, MCF-7 and BT549 cell lines. This evidence concerns the gene ESR1 and breast cancer.