Deal making is facing new uncertainties, as acquisitions that can be perceived as stifling future competition are increasingly called-in for merger review, even in the absence of compulsory notifications. Regulators are reacting to a debate on alleged “underenforcement” – in particular, those known as ‘killer acquisitions’. In jurisdictions with a mandatory turnover threshold regime, residual call-in rights are seen as part of the solution to bridge the enforcement gap.