Why do certain mutations cause cystic fibrosis?

HI

Not sure if this is a complete question, really doesn't make much sense.

Not sure what your question is asking. Cystic fibrosis is an autosomal recessive disease in which the affected person has both recessive genes; a homozygous genotype with cystic fibrosis as the phenotype.

CF is caused by a point mutation in the gene cystic fibrosis transmembrane conductance regulator (CFTR). The most common mutation, deletion F508, is a deletion of three nucleotides that results in a loss of the amino acid phenylalanine (F) at the 508th position on the protein. Although most people have two working copies (alleles) of the CFTR gene, only one is needed to prevent cystic fibrosis. CF develops when neither allele can produce a functional CFTR protein.

Deletion F508-CFTR, which occurs in more than 90% of patients in the U.S., creates a protein that does not fold normally and is degraded by the cell.

Information from wikipedia (with valid sources). Image from www.cff.org.