RRM2 and small cell lung carcinoma: To determine whether the CXCR4/FOXM1/RRM2 axis or the expression pattern that we observed in RNA-Seq and in vitro SCLC models follow a similar pattern in intracardiac xenografts in vivo models or not, we performed expression analysis of CXCR4, FOXM1, and RRM2 using IHC on metastatic liver tissues (as the liver is the most frequent metastatic site of SCLC).