shares will lapse in 3 equal installments upon the achievement of specified performance goals. The Company determined it is not currently probable that these performance goals will be achieved and, therefore, 0 expense has been recorded to date. The fair value of the performance-based shares that could be expensed in future periods, net of estimated forfeitures, is $142,000.
During the six months ended June 30, 2020, holders of options issued under the Company’s equity plans exercised their rights to acquire an aggregate of approximately 130,000 shares of common stock at prices ranging from $2.47 to $3.05 per share. The total proceeds to the Company from these option exercises were $362,000.
In June 2018, the Company's Board of Directors, with shareholder approval, adopted the Employee Stock Purchase Plan, or ESPP. An aggregate of 1,000,000 shares of common stock have been reserved for issuance under the ESPP. On June 30, 2020 and June 30, 2019, 78,000, and 323,000 shares, respectively, were issued to participating employees at a fair value of $1.86 and $1.63 per share, respectively. The fair value of each ESPP award is estimated on the first day of the offering period using the Black-Scholes option-pricing model. The assumptions used in the calculations for each offering period are noted in the table below. The Company recognizes share-based compensation expense equal to the fair value of the ESPP awards on a straight-line basis over the offering period.
| | | | | |
| | June 30, 2020 | | June 30, 2019 | |
Dividend | | NaN | | NaN | |
Volatility | | 85.7% | | 67.3% | |
Risk-free interest rate | | 1.57% | | 2.51% | |
Expected life (years) | | 0.5 | | 0.5 | |
Stock compensation expense related to stock options and restricted stock awards granted under the stock plans and related to the ESPP was $3.4 million and $6.5 million during the three and six months ended June 30, 2020, respectively, compared to stock compensation expense of $2.1 million and $7.1 million for the three and six months ended June 30, 2019, respectively. Stock compensation expense related to the ESPP was $140,000 and $292,000 for the six months ended June 30, 2020 and 2019, respectively. As of June 30, 2020, the estimated fair value of unvested employee awards, exclusive of performance awards, was $23.2 million, net of estimated forfeitures. The weighted-average remaining vesting period for these awards is approximately three years.
Segment Information
During the six months ended June 30, 2020, the Company continued to operate in 1 operating segment, which is the business of development of monoclonal antibody-based anticancer therapeutics.
During the three and six months ended June 30, 2020, 94% and 96%, respectively, of revenues were from Roche, consisting primarily of non-cash royalty revenue, compared to 99% of revenue from Roche in each of the three and six month periods ended June 30, 2019. There were 0 other customers of the Company that generated significant revenues in the three or six months ended June 30, 2020 and 2019.
Recently Adopted Accounting Pronouncements
In November 2018, the FASB issued ASU 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606, which clarifies that certain transactions between participants in a collaborative arrangement should be accounted for under ASC 606 when the counterparty is a customer. In addition, ASU 2018-18 adds unit-of-account guidance to ASC Topic 808, Collaborative Arrangements, in order to align this guidance with ASC 606 and also precludes an entity from presenting consideration from a transaction in a collaborative arrangement as revenue from contracts with customers if the counterparty is not a customer for that transaction. This guidance is effective for annual reporting periods beginning after December 15, 2019, including interim periods within those annual reporting periods. The Company adopted the standard on January 1, 2020, and it did not have a material effect on the Company’s consolidated financial statements.
In June 2016, the FASB issued ASU No. 2016-13, Measurement of Credit Losses on Financial Instruments, to require financial assets carried at amortized cost to be presented at the net amount expected to be collected based on historical experience, current conditions, and forecasts. The ASU is effective for interim and annual periods beginning after December 15, 2019. Adoption of the ASU is on a modified retrospective basis. The Company adopted the standard on January 1, 2020, and it did not have a material effect on the Company’s consolidated financial statements.