GDNF and neoplasm: Recent data have suggested that RET variants or amplification may also occur in ∼1% of breast tumors or metastases (Paratala et al., 2018), while tumor-specific expression of GDNF and ARTN is relatively frequent and can promote autocrine activation of RET downstream signaling (Kang et al., 2009; Kan et al., 2010; Morandi et al., 2013; Nik-Zainal et al., 2016).