(A) Histograms show the sphere volumes of GCSCs derived from the indicated gastric cancer cells. Addition of serine (0.8 mM) could party rescue CSC formation in miR-6778–5p or SHMT1-knockdown gastric cancer cells and could especially rescue CSC formation in Drosha-knockdown gastric cancer cells. (B) Isotopic tracing analysis of relative M+1 and M+2 dTTP in GCSCs derived from Drosha WT and Drosha KD MGC-803 cells cultured in medium with [2,3,3-2H]-serine. (C) Isotopic tracing analysis of M+1 and M+2 dTTP in GCSCs derived from the indicated gastric cancer cells cultured in medium with [2,3,3-2H]-serine. (D) Histograms show the sphere volumes of GCSCs derived from the indicated Drosha WT or Drosha KD gastric cancer cells treated with or without 30 μg/ml of 5-FU (a chemotherapeutic drug for gastric cancer). (E) The miR-6778–5p, SHMT1 and CD44 levels in gastric tumour tissues and their control normal tissues and 5-FU sensitive or resistant gastric tumour tissues were evaluated by qRT-PCR.