GCN Circular 29275
J. Mao, X. Ding, X.-L. Zhang, and J.-M. Bai (YNAO) report: We observed the field of GRB 210104A (Troja et al. GCN 29233) with the 2.4-meter optical telescope at Gao-Mei-Gu (GMG) station of Yunnan Observatories. The observation began at UT 13:00:11, 4, Jan. 2021, about 1.5 hours after the trigger. We clearly detected the source, and the preliminary magnitude was measured to be R~17.6. Further analysis of the observations during the night is ongoing.