SHMT1 and cancer: In this follow-up study, we investigate the effect of CK2 inhibition on DHFR and TS expression in two different cancer cell lines, A549 (adenocarcinomic human alveolar basal epithelial cells) and CCRF-CEM (human acute lymphoblastic leukemia cells), while the expression of two forms of serine hydroxymethyltransferase, cytosolic (SHMT1) and mitochondrial (SHMT2), was studied in the CCRF-CEM cell line.