If you're curious to the biophysics of this, the delta F508 gene is actually functional, but rejected by the endoplasmic reticulum because the ER fails to fold

If you're curious to the biophysics of this, the delta F508 gene is actually functional, but rejected by the endoplasmic reticulum because the ER fails to fold it correctly. It's possible that you have milder symptoms because your protein folding machinery is either more capable of folding the gene correctly, or alternatively, your protein checking machinery is more lenient.