Yellow cards expire ahead of semi-finals

All yellow cards accrued by those teams still in the UEFA Champions League and UEFA Europa League have expired ahead of the semi-finals.

As of the start of 2014/15, yellow cards accumulated in UEFA club competitions from the beginning of the group stage are wiped out upon completion of the quarter-finals. That means all players left in the competition now have a clean slate, with the exception of those that incurred a suspension in the quarter-final second legs and will miss the semi-final first legs, namely:

Sami Khedira (Juventus, three yellow cards)
Nick Viergever (Ajax, three yellow cards)
Joël Veltman (Ajax, red card)

The official 2016/17 UEFA Champions League regulations state:

49.03 Single yellow cards and pending suspensions are always carried forward either to the next stage of the competition or to the UEFA Europa League in the current season.

49.04 Exceptionally, all yellow cards and pending yellow-card suspensions expire on completion of the play-offs. They are not carried forward to the group stage. In addition, all yellow cards expire on completion of the quarter-finals. They are not carried forward to the semi-finals.

49.05 Cautions and pending yellow-card suspensions from the competition or the UEFA Europa League expire at the end of the season.

The official UEFA Europa League regulations can be found here (see Article 48).