Based on the expression patterns of neuroendocrine markers such as neural cell adhesion molecule 1 (NCAM1/CD56), chromogranin A (CHGA), synaptophysin (SYP), and gastrin-releasing peptide (GRP) in tumor cells, SCLC can be categorized into high neuroendocrine subtypes (NE) and low/no neuroendocrine (low NE/non-NE) [18,19]. Here, GRP is linked to small cell lung carcinoma.