Apolipoprotein B (APOB) is the primary apolipoprotein of low density lipoproteins (LDL),  or "bad cholesterol", which is responsible for carrying cholesterol to tissues.  APOB "unlocks" the doors to cells and thereby delivers cholesterol to them.

High levels of APOB can lead to plaques that cause heart disease.

Recent research shows that levels of APOB are a better indicator of heart disease risk than total cholesterol or LDL.

References

Transgenic mice that overexpress mouse apolipoprotein B. Evidence that the DNA sequences controlling intestinal expression of the apolipoprotein B gene are distant from the structural gene. J Biol Chem. 1996 May 17; 271(20): 11963-70; Knockout of the Mouse Apolipoprotein B Gene Results in Embryonic Lethality in Homozygotes and Protection Against Diet-Induced Hypercholesterolemia in Heterozygotes. Proc Natl Acad Sci USA. 1995 Feb 28; 92(5): 1774-8;