CXCL12 and neuroblastoma: While we did observe decreases in CXCR4 surface levels over increasing time and cell confluency (Fig. 3), results from ELISA experiments performed on media from confluent cultures of neuroblastoma cells revealed negligible levels of SDF-1 (Fig. 5), this was despite the detection of SDF-1 transcripts (data not shown) and protein (Fig. 6) inside the cells.