The Internal Revenue Service (IRS) said it is sending letters to approximately 100,000 tax return preparers who failed to follow the requirements of its new Preparer Tax Identification Number (PTIN) registration program while preparing returns in 2011. Launched last fall in an effort to increase its oversight of the tax return preparation industry, the program requires all paid tax-return preparers to obtain a PTIN number and, when required to do so, sign their names and include their PTINs on the returns and refund claims they prepare for compensation. About 712,000 tax preparers have registered and obtained PTINs.