with some genes you will get 50% of RNA with a het, some not.  as Dr. amberg noted, one possibility is monoallelic expression, but I dont think that is very common on a strict monoalleleic expression basis.  Other reasons for greater than 50% are feedback that either cause more expression from the single allele or better stability of the mRNA which shows up as more mRNA.  If there is monoallelic expression for this gene then again the feedback could make it more likely that the intact allele is expressing.  

Hi Hajime, not necessarily inactivation of one allele corresponds to a halving of the RNA levels. Although most autosomic gene expression is biallelic, many genes respond to one allele inactivation increasing the expression from the remaining allele. This really depends on the gene. Anyway small changes in expression levels for some genes are sufficient to produce a phenotype. This is not unusual and should not puzzle you. Sometimes a large window of expression is tolerated for "normality", other times not, slight changes can have dramatic effects. The latter is mostly true for signaling molecules and for transcription factors, the levels of which are quite tightly regulated due to their powerful effects.

If you want to have an idea of the global changes induced by the knock out of one allele of your gene, you should go at least for an RNA Seq (or microarray) analysis of the BM cells from the wt and the +/-. Even better would be to look at differential protein expression and protein modification (for example with an iTRAQ-2DLC-MS/MS approach, as well as, for example, a TiO2-MOAC enrichment followed by LC-MS/MS, respectively, if you have access to a Mass Spec facility) to have the real picture of what is changing in the mutated cells.

Dear Dr. Amberg, Dr. Fiering, and Dr. Boyl

Thank you for your kind and great answer.

I will check into that possibilities.

It depends on whether your mRNA is getting degraded either by Nonsense-mediated mRNA decay (NMD) or other degradation pathway. If so happens then you should be expecting a 50% reduction in your mRNA level in most cases.