Theranos, the blood-testing startup that has been roiled by a Wall Street Journal investigation into its business practices over the past year, said Wednesday evening that it would close its labs and Theranos Wellness Centers and lay off 340 workers.

Founder and Chief Executive Elizabeth Holmes said in a letter posted publicly that the company would focus exclusively on a new technology it showed off in August, a so-called MiniLab that allows for testing of blood on-site.