Do all of your clones exhibit reporter gene (lacZ, gfp, venus, depending on vector) expression?
Not always. Unless the reporter gene has an IRES preceding it (as in pMS1 and UPATrap) expression will only result if the insertion of the reporter gene ends up being in the correct reading frame with the gene that it was inserted into. Expression also depends on when your gene is expressed. Most gene trap vectors require expression in undifferentiated ES cells to trap the gene. The polyA trap vectors have changed this requirement and allow for unexpressed genes to also be trapped. If a trapped gene is only expressed later on in development, or in only certain types of cells, expression could be seen upon differentiation of the cells into specific lineages that do express the gene.
