Loss of heterozygosity during the development of tumours is exactly as it sounds.   It does not always involve deletion (contrary to what Wikipedia says!). 

You'll know that heterozygous means having  two different alleles of a gene.  This can involve partial deletion of one allele, or it also can involve a mutation or variant of one copy.   If an organism or tissue starts off heterozygous, then LOH can happen within a tumour (or a subclone within a tumour); it means that the cells become no longer heterozygous for that gene.  This often indicates that  the lost allele was a functional copy of a tumour suppressor gene, and the allele that remains is a dysfunctional or nonfunctional copy.  So loss of the WT allele can promote tumour growth.  If it involves deletion of part of a chromosome, LOH may alternatively mean that there was a tumour suppressor gene nearby on the chromosome, rather than the one you are looking at. 

LOH can happen in various ways.  It may just be loss of the WT copy, or there may be substitution -- a chromosomal rearrangement(s) giving two (or more) copies of the chromosome (or chromosomal band) carrying the deficient copy.  It can even be that the WT copy happens to undergo the same mutation as the other copy.

   As Harald said, there are various ways in which gene dysfunction can affect protein expression.  So in some cases a protein will be expressed from the remaining mutant copy(ies) following LOH, or in other cases not.  Expression may even be greater - mutant p53 is often more highly expressed than normal p53, for instance. 

I didn't think that LOH meant no protein, but a lot of my articles seem to be assuming that. Thank you for all your help!!! 

@  Dorothy C Bennett,

I am from plant field. I am wondering if this term LOH is specifically used in cancer field? Not in other field, such as in plant science.

Hi Yuan-Yeu.   I am not a population geneticist and am not sure how widely the actual term is used there.  However the process of LOH can also happen in populations, like animals or plants:  for example through inbreeding, where variant alleles are lost and many loci become homozygous.  Or in an individual that undergoes a germline or somatic deletion event, leaving only one copy of a gene or a chromosomal region.

Dear Dorothy,

Thanks for your explanation.

Yes, in plant breeding field, when 'lost of heterozygosity' is mentioned, we automatically think of the gain of homozygosity, such as in inbreeding a plant. Inbreeding is used to produce inbred. Inbred are then used for hybrid seeds production. Due to 'hybrid vigor' (or heterosis), usually hybrid plants have better performance (ex. higher yield......). A lot of crop are produced using hybrid seeds these days.

I looked into LOH definition (in cancer field). It simply means that there are 2 copies of good genes (ex. cancer repress gene) in somatic cells (ex. human, one from father, one from mother), and one day, one copy of them becomes mutated (they call that 'the first hit'). After that, there is still a good copy left for expression. So, the individual is probably ok. If unfortunately, the left good copy is also mutated later (they call it 'the second hit'), then the individual will have trouble. So, I think that when these 3 things [LOH-first hit-second hit] are mentioned together, it most likely used in 'cancer field'. In plant science, I have not heard these terms [LOH-first hit-second hit] used together. Even it [LOH-first hit-second hit] occurs in plant somatic cells, the plants probably won't die anyway.