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http://www.cyclingnews.com/news/uae-team-emirates-wear-zipless-jersey-at-tour-de-france/
Seems it's the latest in Lightweight/aero clothing.
Aero because the riders cannot unzip it, it seems.
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There's a write up on it here. Basically for aero gains but i'd say it's a bitch to get on.
https://www.bikeradar.com/road/news/article/uae-team-emirates-zipless-jersey-tour-de-france-52633/
Edit: page refresh fail
I didnt realise Dan's (and presumably the rest of the teams') jersey didnt have a zip on it. Just noticed it there. Must be to do away with the zip, looked kinda strange though