BTC.sx, who offers trading in bitcoin-based derivative instruments, has halted trading due to the cessation of MtGox operations. Without explicitly mentioning MtGox, their update reads as follows:
Thank you for your continued support of BTC.SX, we really appreciate your custom. Unfortunately, due to the current closure of our main exchange partner, we regret we will be unable to take any further orders and our site will be suspended until further notice. All customer balances are secure and we will honour any withdrawal requests. For open positions, these will be liquidated and deposits will be refunded in full.
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We apologise for any inconvenience caused, we are working hard to integrate new exchanges and offer our service again as soon as possible.
We will keep you informed of any update, all refunds will be processed in due course, thank you for your patience during this time.
Plus500 had allowed existing positions based on MtGox to remain open, only closing off the instrument to new positions. In this case, however, there’s not much of a choice as MtGox has disappeared entirely.