Composite colorectal carcinomas are rare. There are amodest number of cases in the medical literature,with even fewer cases describing composite carcinoma with neuroendocrine and squamous components. There areto our knowledge no reports of composite carcinomamolecular alterations. We present a case of composite carcinoma of the splenic flexure in a 33 year-old Cau-casian male to investigate the presence and prognos-tic significance of molecular alterations in rare coloniccarcinoma subtypes. Formalin-fixed paraffin-embedded(FFPE) tissue was hematoxylin and eosin- and mucicar-mine-stained according to protocol,and immuno-stained with cytokeratin(CK) 7,CK20,CDX2,AE1/AE3,chromo-granin-A and synaptophysin. DNA was extracted from FFPE tissues and molecular analyses were performed according to lab-developed methods,followed by capil-lary electrophoresis. Hematoxylin and eosin staining showed admixed neuroendocrine and keratinized squamous cells. Positive nuclear CDX2 expression confirme dintestinal derivation. CK7 and CK20 were negative.Neuroendocrine cells stained positively for synaptophysin and AE1/AE3 and negatively for chromogranin and mucicarmine. Hepatic metastases showed a similar immunohistochemical profile. Molecular analysis revealeda G13D KRAS mutation. BRAF mutational testing wasnegative and microsatellite instability was not detected.The patient had rapid disease progression on chemotherapy and died 60 d after presentation. Although theG13D KRAS mutation normally predicts an intermed iateoutcome,the aggressive tumor behavior suggests other modifying factors in rare types of colonic carcinomas. There are amodest number of cases in the medical literature, with even fewer cases describing composite carcinoma with neuroendocrine and squamous components. There areto our knowledge no reports of composite carcinomamolecular alterations. We present a case of composite carcinoma of the Formalin-fixed paraffin-embedded (FFPE) tissue was hematoxylin and eosin- and mucicar-mine-stained according to the protocol, and immuno-stained with cytokeratin (CK) 7, CK20, CDX2, AE1 / AE3, chromo-granin- A and synaptophysin. by capil-lary electrophoresis. Hematoxylin and eosin staining showed mixed with neuroendocrine and keratinized squamous cells. Positive nuclear CDX2 expression confirme dinte Stable derivation. CK7 and CK20 were negative. Neuroendocrine cells stained positively for synaptophysin and AE1 / AE3 and negatively for chromogranin and mucicarmine. Hepatic metastases showed a similar immunohistochemical profile. Molecular analysis revealed G13D KRAS mutation. BRAF mutational testing was negative and microsatellite instability was not detected. the patient had rapid disease progression on chemotherapy and died 60 d after presentation. Although the G13D KRAS mutation normally predicts an intermediatedatecome, the aggressive tumor behavior suggests other modifying factors in rare types of colonic carcinomas.