Merrimack is conducting a research study to see if an experimental drug, called MM-093, is safe and effective in the treatment of psoriasis. MM-093 is a genetically engineered version of a naturally occurring protein called alpha fetoprotein (AFP). Adults normally have very small amounts of AFP in their bloodstream. However, during pregnancy, AFP levels in both the mother and the fetus are much higher than normal. It has been observed that some women with psoriasis and other autoimmune diseases such as rheumatoid arthritis have fewer symptoms during pregnancy, particularly during the third trimester. At this time, the levels of AFP in the blood of the mother and fetus are the highest. This observation led Merrimack to begin examining MM-093 as a potential treatment for psoriasis. This study is designed to further test the safety and effectiveness of MM-093 in patients with psoriasis.