B2M and neoplasm: There are two possible explanations for the elevated serum β2-M in patients with colorectal cancer: first, the assembly of β2-M and class I heavy chain on the tumor cell membrane does not result from conformational changes in the heavy chains, leading to reduced expression of class I antigen on tumor cells and the accumulation of free β2-M in the serum; secondly, there is increased renewal of tumor cells in patients with colorectal cancer, resulting in increased shedding of β2-M from tumor cells and subsequently elevated serum levels.