stock to transfer their shares of common stock. Our Board may authorize the issuance of preferred stock with voting, conversion, dividend, liquidation and other rights that may adversely affect the rights of the holder of our common stock.
Pursuant to the terms of that certain Unit Purchase Agreement, dated May 5, 1998 (the “UPA”) we issued and sold a total of 149,960 shares of common stock (the “Put Shares”) at a price of $128.00 per share. Under the UPA, the initial purchasers of the Put Shares (the “Put Holders”) have the right to require us to repurchase the put shares (the “Put Right”). In order for the Put Right to be exercised by any Put Holder all of the following must occur: (1) we liquidate, dissolve or wind up our affairs pursuant to applicable bankruptcy law, whether voluntarily or involuntarily; (2) all of our indebtedness and obligations, including without limitation the indebtedness under our outstanding notes, has been paid in full; and (3) all rights of the holders of any series or class of capital stick raking prior and senior to the common stock with respect to liquidation have been satisfied in full. We may terminate the Put Right upon written notice to the Put Holders if the closing sales price of our common stock exceeds $256.00 per share for the 20 consecutive trading days prior to the date of notice of termination. Because the Put Right is not transferable, in the event that a Put Holder has transferred Put Shares since May 5, 1998, the Put Right with respect to those Put Shares has terminated. As a consequence of the Put Right, in the event we are liquidated, holders of shares of common stock that do not have a Put Right with respect to such shares may receive smaller distributions per share upon our liquidation than if there was no Put Right outstanding. As of the date of the Annual Report on Form 10-K of which this Exhibit 4.21 is a part, we had repurchased or received documentation of the transfer of 49,993 Put Shares and 4,472 of the Put Shares continued to be held in the name of the Put Holders. We cannot determine at this time what portion of the Put Rights of the remaining 95,494 Put Shares have terminated.
As of the date of the Annual Report on Form 10-K of which this Exhibit 4.21 is a part, 42,257,456 shares of common stock are issued and outstanding and 34,144,163 shares of common stock were reserved for the issuance upon the exercise of outstanding warrants and options to purchase common stock, outstanding restricted stock units, the conversion of Series A convertible preferred stock (“Series A”) and Series B1 redeemable convertible preferred stock (“Series B1”), shares required to be reserved under the Purchase Agreement with Lincoln Park Capital Fund, LLC, which was amended on September 2, 2020, and shares available for grant under our 2013 Stock Incentive Plan and shares available for purchase under our 2017 Employee Stock Purchase Plan.
Preferred Stock Convertible Into Common Stock
We are authorized to issue 5,000,000 shares of preferred stock, of which 1,500,000 has been designated Series A, 277,921 has been designated Series B1, 98,685 has been designated Series B2 redeemable convertible preferred stock (“Series B2”), 82,814 has been designated Series B3 redeemable convertible preferred stock (“Series B3”), and 82,814 has been designated Series B4 redeemable convertible preferred stock (“Series B4” and, together with Series B1, Series B2, and Series B3, “Series B Preferred Stock”).
Shares of Series A, in whole or in part, at the option of the holder, are convertible into fully paid and nonassessable shares of common stock at $272.00 per share, subject to adjustment. Each share of Series B Preferred Stock is initially convertible into 100 shares of common stock. Shares of Series B1 and Series