MK:
The 1K/PremEx checkin is to the right of the First Class/Premier checkin over by door #1. It's only open during "normal hours" (i.e., weekdays, opens about 5:30am and closed around 7pm, not as sure about weekends) It's kind of out of the way, but once you know it's there, you never go back! There is usually a UA person standing there to make sure your 1K/PremEx also. Now, if I could only use the 1K line at BOS...
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Regards,
- Anna