A2B5 is a cell surface ganglioside epitope expressed in developing thymic epithelial cells, oligodendrocyte progenitors and neuroendocrine cells. I haven't found its mRNA sequence yet.
A2B5 is an antibody recognizing an epitope of the ganglioside molecule, which is specific to CNS cells. There are many gangliosides subtypes, if you are intrested to as what is specific epitope sequence for this Ab you might have to contact the company and see if they would provide you with this information. Otherwise you could experimentally narrow down the sequence of the epitope to this Ab. This would then allow you to search for mRNA sequence of this epitope.
The other way is to just do literature search (depending on what you intend to do) and look at gangliosides expression. Although I am not familiar with this field and don't know what is out there.