TACR1 and cancer: In contrast to the NK-1R, SP is not essential for cancer cells, because the undecapeptide can be released from nerve cells (the number of SP-positive nerves is related to cancer differentiation) [65] and from immune cells (e.g., located in the tumor microenvironment), and SP can reach cancer cells from the blood and/or the NK-1R can be also activated by hemokinin-1, a peptide that belongs to the tachykinin family of peptides and that, in addition to SP, is a peripheral natural ligand of the NK-1R.