We investigated the molecular and clinical profile of five boys carrying the fragile X messenger ribonucleoprotein 1 (FMR1) mutation and who suffered from the effects of prenatal alcohol exposure. Fragile X syndrome (FXS) testing was performed using PCR and Southern Blot analysis, and fragile X messenger ribonucleoprotein protein (FMRP) expression levels were measured by Western blot analysis. Clinical evaluation included cognitive functions, adaptive skills, autism phenotype, and severity of behavior measures. Fetal Alcohol Spectrum Disorder (FASD) was also assessed. Five adopted male siblings were investigated, four of which (cases 1, 2, 3, and 4) were diagnosed with FXS, FASD, and ASD, and one, the fraternal triplet (case 5), was diagnosed with FASD and ASD and no FXS. The molecular profile of case 1 and 2 showed the presence of a hypermethylated full mutation (FM) and the resulting absence of FMRP. Cases 3 and 4 (identical twins) were FM-size mosaics (for the presence of an FM and a deleted allele), resulting in 16% and 50% FMRP expression levels, respectively. FMRP expression level was normal in case 5 (fraternal twin). Severe behavioral problems were observed in all cases, including aggression, tantrum, self-harming, anxiety, and defiant behavior, due to different mutations of the FMR1 gene, in addition to biological exposure, home environmental factors, and potentially to additional background gene effects.