NCBI gene data

Question

[chandrakala.m]

In NCBI gene search, there are several variants of a gene represented in alphabet (Variant a, b, c...). these variants code for protein isoforms. but, there are few variants called X1,X2 and so on. what do they represent and how do they function? Please clarify on how are they different from a,b,c variants? Thank you.

[GenoMax]

Please post examples when referring to any ID's.