The portion of Bank of China's US$9.9 billion initial public offering open to institutional investors is five to six times oversubscribed, the Wall Street Journal reported, citing people familiar with the offering. The retail tranche of the IPO, the world's largest so far this year, is due to start Thursday. By comparison, institutional interest in China Construction Bank's IPO was nine times the shares on offer, while Bank of Communications was 20 times subscribed. Bank of China will begin trading on June 1, priced at between 1.9 and 2.2 times its book value. The IPO shares are equal to about 10.5% of the bank's enlarged share capital, but this could rise to 11.89%.