The most common cause of beta thalassemia
The  mutations  responsible  for  β-thalassemia  disrupt β-globin synthesis in several different ways
Mutations  leading  to  aberrant  RNA  splicing  are the most common cause of β-thalassemia. Some of these mutations disrupt the normal RNA splice junctions; as a result, no mature mRNA is made and there is a  complete  failure  of  β-globin  production,  creating  β 0 .
Other mutations create new splice junctions in abnormal positions—within  an  intron,  for  example.  Because  the normal splice sites are intact, both normal and abnormal splicing occurs, and some normal β-globin mRNA is made.

These alleles are designated β + .

Some  mutations  lie  within  the  β-globin  promoter  and lower  the  rate  of  β-globin  gene  ranscription.  Because some normal β-globin is synthesized, these are β +  alleles.
Other  mutations  involve  the  coding  regions  of  the  β-globin  gene,  usually  with  severe  consequences.  For example, some single-nucleotide changes create termination  (“stop”)  codons  that  interrupt  the  translation  of β-globin mRNA and completely prevent the synthesis of β-globin.
Mutations  leading  to  aberrant  RNA  splicing.
Mutations  lie  within  the  β-globin  promoter  and lower  the  rate  of  β-globin  gene  transcription.
Mutations  involve  the  coding  regions  of  the  β-globin  gene
All of the above are equally common