Mark and Xiumin have been known to be close since before Mark's debut days. In one photo of Mark and Xiumin with their backs to the camera, Mark wrote, "With Xiumin sunbaenim, at the SMTOWN Olympics!", while in another selca with Xiumin, he wrote, "I met the cool Xiumin sunbaenim at the SMTOWN workshop~ ^^ It was good to see him, and a fun time." 

Fans even noted how tall Mark has gotten since his pre-debut days in S.M.ROOKIES, when he was shorter than Xiumin. Meanwhile, staff and artists who attended the SMTOWN workshop in Jeju island returned to Seoul on June 21. 

It looks like they had a lot of fun!

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[#??] ?? ??? ????? SMTOWN ????? ????~~^^ ?? ??? ??? ??????? #SMTOWN #JEJU

A post shared by NCT 127 Official Instagram (@nct127) on Jun 20, 2017 at 7:19am PDT