beta-Thalassemia (thal) is the most common recessive inherited disorder in 
Mediterranean populations. It is estimated that the frequency of this disease in 
the Moroccan population is between 1.5 and 3.0%. Severe forms of homozygous 
thalassemia cases require expensive and technically demanding curative (bone 
marrow transplantation) or palliative (chronic transfusion/chelation) therapies. 
The -158 (C-->T) polymorphism of the (G)gamma-globin gene (XmnI polymorphism) is 
known to ameliorate the severity of the disease because of it strong association 
with an increased production of fetal hemoglobin (Hb F). Among the many known 
mutations in Morocco, six are common [codon 39 (C-->T), frameshift codon (FSC) 8 
(-AA), IVS-II-745 (CG), FSC 6 (-A), -29 (A-->G) and IVS-I-1 (G-->A)]. In this 
study, we have investigated, in 82 Moroccan beta-thalassemic chromosomes, the 
correlation between the six common mutations and the XmnI polymorphism using the 
Fisher exact test. The XmnI polymorphism was divided into two categories, (XmnI 
[+] and XmnI [-]) and the six common Moroccan mutations into two groups (group I 
with FSC 8 and group II without FSC 8). Correlation was carried out between the 
XmnI [+] category and the six common mutations individually that showed that 68% 
of chromosomes in the XmnI [+] category had the FSC 8 (-AA) mutation. The 
results reported here show that there is a positive correlation between the XmnI 
polymorphism and FSC 8 mutation in linkage with haplotype IV [- + - + + - +] (p 
<10(-5)). In conclusion, molecular determination of genetic markers in early 
childhood will help to identify candidates for pharmacological Hb F switching by 
hydroxyurea (HU). In the Moroccan population, a good response to HU treatment 
should be suspected in cases with the -158 (C-->T) polymorphism in linkage with 
haplotype IV and internal beta-globin gene framework 3.