Spark Therapeutics (ONCE)  lists publicly on Friday, joining companies like uniQure (QURE), bluebird bio (BLUE), and Avalanche Biotechnologies (AAVL) as yet another emerging gene therapy player. It's a highly watched IPO in-part because the company recently partnered with Pfizer (PFE) on a therapeutic for hemophilia B, and Spark has phase III data due in the second half of the year for a rare genetic blinding condition. Here's your 30-second primer for the IPO.

• Lead candidate SPK-RPE65 targets a group of rare blinding conditions known as inherited retinal dystrophies (IRDs), which are caused by autosomal recessive mutations in the RPE65 gene. Spark estimates that there are approximately 3,500 individuals with RPE65-mediated IRDs between the U.S. and the major European markets.
• A follow-on IRD candidate, SPK-CHM, recently entered a phase I/II dose-escalation study for the treatment of choroideremia (CHM). CHM is an IRD that manifests as night blindness and a reduction in visual field in males. Spark estimates that CHM affects 12,500 people in the US and EU.
• The Pfizer-partnered SPK-FIX program for the treatment of hemophilia B will enter a phase I/II trial in the first half of the year. Spark is entitled to a $20.0 million upfront payment and up to $260.0 million in aggregate milestones, as well as low-teen royalties on sales. Spark puts the global hemophilia B market at 28,000 patients.
• All of the above programs use a modified adeno-associated virus (AAV).
• Spark is selling 7 million shares of common stock at $23 in the public offering, with 1.05 million set aside for the underwriters of the offering. At max, the company will gross $185 million, prior to expenses.
• The outstanding sharecount after the offering, including the underwriter's over allotment, is 24,540,367 shares. This excludes:
  • 2,264,497 shares issuable on exercise of stock options outstanding as of January 15, 2015 (average exercise price of $4.52)
  • 209,500 shares reserved as of January 15, 2015 for future issuance under the company's 2014 equity incentive plan
  • 1,830,000 additional shares available for future issuance under the 2015 stock incentive plan
  • 220,000 additional shares available for future issuance under the 2015 employee stock purchase plan
  • ONCE will have a fully diluted sharecount of 29 million.
• At $25, ONCE has a $613 million market cap. 
• At $30, ONCE has a $736 million market cap.
• At $35, ONCE has a $893 million market cap.
• At $45, ONCE has a $1.1 billion market cap.