the $81 billion merger of Medtronic and Covidien in 2015. The US tax code's anti-avoidance rules prohibit a US company from creating a new "legal" headquarters in Ireland while its main business is in the US (known as a "self-inversion"). Until April 2016, the US tax code would only consider the inverted company as foreign (i.e. outside the U.S. tax-code), where the inversion was part of an acquisition, and the Irish target was at least 20% of the value of the combined group. Although the Irish target had to be over 40% of the combined group for the inversion to be fully considered as outside of the U.S. tax-code. Once inverted, the US company can use Irish multinational BEPS strategies to achieve an effective tax rate well below the Irish headline rate of 12.5% on non–U.S. income, and also reduce US taxes on US income. In September 2014, Forbes magazine quoted research that estimated a US inversion to Ireland reduced the US multinational's aggregate tax rate from above 30% to well below 20%. In July 2015, The Wall Street Journal noted that Ireland's lower ETR made US multinationals who inverted to Ireland highly acquisitive of other US firms (i.e. they could afford to pay more to acquire US competitors to re-domicile them to Ireland), and listed the post-inversion acquisitions of Activis/Allergan, Endo, Mallinckrodt and Horizon. In July 2017, the Irish Central Statistics Office (CSO) warned that tax inversions to Ireland artificially inflated Ireland's GDP data (e.g. without providing any Irish tax revenue). U.S. inversions Share of inversions , Bloomberg ranks Ireland as the most popular destination for U.S. tax inversions, attracting almost a quarter of the 85 inversions since 1983: List of inversions , the table below is the Bloomberg list of the 21 U.S. corporate tax inversions to Ireland (note, Bloomberg lists U.S. corporate tax inversions by order of the year in which they first left the U.S. which is used below). The table also includes some known aborted U.S. tax inversions to Ireland. All of the U.S. multinationals listed below are listed on stock exchanges and the Market Capitalisation is as per 21 November 2018. "Spin-off inversions" and "self-inversions" happen outside of the U.S. (e.g. the corporate/corporate parent had previously left the U.S.), and thus do not need an acquisition of an Irish—based corporate to execute the move to Ireland, as per U.S. tax code requirements (see above).