Background: Mastomys ECL cell proliferation is initially gastrin-driven, but once neoplasia develops, cells become gastrin-autonomous. We hypothesized that CCN2, a mitogenic growth factor, may regulate ECL cell proliferation. Methods: CCN2 expression levels were identified in a Mastomys GeneChip database, in FACS sorted normal and tumor ECL cell preparations (real-time PCR) and in mucosal and ECL cell preparations (immunohistochemistry). Short-term cultured cells were stimulated with either CCN2 or CCN2+EG, proliferation measured (MTT) and the effects of PD98059 assessed. CCN2 transcript and protein was then examined in clinical gastric carcinoids. Results: ccn2 transcript was upregulated in tumor samples compared to normal mucosa (+2.36-fold, p<0.01). ccn2 was not expressed in FACS prepared (>98% pure) normal ECL cells but was elevated in tumor ECL cell fractions (41.3-fold±10.7). Immunostaining confirmed that CCN2 protein was present in ECL tumors but not in normal ECL cells. Neither CCN2 nor CCN2+EGF stimulated normal ECL cell proliferation. CCN2 stimulated tumor proliferation (EC₅₀ ~0.01 ng/ml); EGF significantly augmented (p<0.01) CCN2-induced tumor cell proliferation (EC₅₀ = 20 pg/ml). PD98059 inhibited CCN2-induced proliferation (-12±3%, p<0.05) and ERK1/2 phosphorylation (-34±5%, p<0.05) in tumor cells. In clinical samples, both CCN2 transcript and protein were elevated in gastrin-autonomous carcinoids (p<0.02) compared to normal mucosa. Conclusion: CCN2 may be a proliferative regulator of Mastomys ECL neoplastic proliferation once these cells become autonomous of gastrin regulation. Identification of CCN2 in gastric carcinoid tissue may be useful both as an indicator of ECL cell transformation and define gastrin-autonomy, a criteria of gastric carcinoid malignancy.