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The Atacama Pathfinder EXperiment (APEX) is a 12 m antenna now operating at the Llano Chajnantor on the Atacama Desert in Northern Chile at 5 100 m altitude. APEX will be equipped with single-pixel heterodyne receivers covering 211 - 1500 GHz frequency range. We present a sideband separation (2SB) mixer using superconducting-insulator-superconductor (SIS) junction for the APEX band 2, 275-370 GHz. The 2SB mixer is based on a previous development of a double sideband (DSB) mixer, which is currently installed at the APEX telescope. This DSB receiver has a noise temperature of about 40-50 K across the band, and, as installed on one of the best site on the Earth, yields total DSB system noise temperatures of about 100 K for excellent weather.