Here we present a challenging case of a hepatic flexure colon tumor of 61-year-old woman with no primary lesion of lung cancer. Immunohistochemistry was applied and 50 genes were analyzed by next-generation sequencing (NGS) technology. The tumor contained medium to large size neoplastic cells with evident nucleoli to be diagnosed poorly differentiated neuroendocrine predominantly large cell carcinoma of colon [G3: World Health Organization (WHO) 2010] (pT3 N0: 7th edition pTNM). Cytokeratin (CK) AE1÷AE3 staining was predominantly membranous with partial distribution in “dot-like” pattern in perinecrotic cancer fields to be reminiscent of small cell carcinoma. Ki67 labeled over 90% of cancer cells with partial positive nuclear staining for thyroid transcription factor-1 (TTF-1). Using NGS, the following mutations were detected: nonsense mutations in four tumor suppressor genes [APC R1114X (molecular argument that the cancer was a primary tumor of colon), TP53 R213X, RB1 E137X and FBWX7 R393X & S282X], mutations in three receptor tyrosine kinases (RET A919V of high transforming activity, EGFR E114K and FLT3 L601I) well known as oncogenes.