Double immunofluorescence staining with the newly formed neuron-specific marker neurofilament-L (NF-L) and the sympathetic nerve marker tyrosine hydroxylase (TH) revealed that 4T1 mammary tumors displayed denser sympathetic innervation 5, 7, and 9 days after tumor development (Figure 2, B and C, and Supplemental Figure 4). Here, TH is linked to breast cancer.