most tumor types. There are a few exceptions, for example, an increase in ID1 expression in brain cancer is correlated with a better prognosis, while a decrease in ID4 expression in colon and rectal cancers is linked to a poorer prognosis. ID proteins can bind E-proteins, preventing them from binding bHLH proteins and halting transcription, a case often seen in cancerous phenotypes. Subtypes Humans express four types of Id proteins (called ID1, ID2, ID3, and ID4). A recent publication in Cancer Research (August 2010) has shown that ID1 can be used to mark endothelial progenitor cells which are critical to tumour growth and angiogenesis. This publication has demonstrated that targeting ID1 resulted in decreased tumour growth. Therefore, ID1 could be used to design a novel cancer therapy. Perk, Iavarone, and Benezra, (2005), reviewed fifteen studies and compiled a list of the phenotypic effects of each ID gene when knocked out in mice. When ID1 was knocked out, a defect in T-cell migration was seen. A knockout of ID2 showed that 25% of mice died perinatally, and those born lacked lymph nodes and showed defects in mammary proliferation. Generally, normal development was seen in mice with an ID3 knockout, but they did have a defect in B-cell proliferation. Neural defects and premature differentiation were seen in mice lacking ID4. Knockout of both ID1 and ID3 resulted in embryonic lethality due to brain hemorrhages and abnormalities in cardiac development. References External links Enzyme inhibitors Protein families