| One of the ways that cells can become cancerous is by activation/inactivation of certain genes involved (usually) in cell cycle and growth control (proto-oncogenes and tumor suppressor genes). Quick biology recap: chromosomes come in pairs - in humans, 23 pairs = 46 chromosomes in all. When somebody has a genetic predisposition to cancer, it often means that one chromosome's copy of a relevant gene is "cancer-causing", but the other copy is not. "Loss of heterozygosity" (LOH) occurs when the "safe" copy is mutated into a cancer-causing version, so both are the same. The cell is then potentially a tumour cell, growing and dividing abnormally.
A particularly tragic and well-documented cancer invoking LOH is retinoblastoma, which can appear in early childhood as a result of LOH in the tumour suppressor gene RB-1 in predisposed individuals before birth. Since only one mutation need occur to initiate the tumour, early childhood retinoblastomas are often bilateral (both eyes) and multifocal (multiple tumour origins). Retinoblastoma also occurs later in life, more sporadically as a result of TWO mutations, in both copies of RB-1. | 
