Transposing Columns to Rows- MILLIONS of Records

I have a HUGE table that I need to do this from, so I need the most efficient way to do this in MySQL.
Can you please help me with this project?
Sample data:
Member DOS Dx1 Dx2 Dx3 Dx4 Dx5
12345 1/1/2011 12142 12345 65657 5657 568
56484 3/5/2011 568 56785 5695 575 168
56872 2/12/2011 567 567

What I need to see is this:
Member DOS DX Seq
12345 1/1/2011 12142 Dx1
12345 1/1/2011 12345 Dx2
12345 1/1/2011 65657 Dx3

and so forth. Only showing those Dx where not null- so for 56872, we'd only see Dx1 and Dx2, but for the other 2, we'd see records for all 5 Dx.

Is anyone able to help me?
Thank you.