Big Week for IPOs

Today 4 IPOs began trading, one day after 3 new issues entered the market.

Here are today's IPOs:

• CafePress ($PRSS) - Owns e-commerce sites where customers create, buy, and sell personalized products. Priced at $19.

• Millenial Media ($MM) - The 2nd largest mobile advertising platform in the US. Priced at $13.

• Rexnord ($RXN) - Manufactures gears, bearings, and water management products. Priced at $18.

• Merrimack Pharmaceuticals ($MACK) - Uses a network biology system to develop cancer treatments. Priced at $7.