In sickle cell anaemia, where amino acid modification occurs in  $\mathrm{\beta }$-chain?

In sickle cell anemia, a thymine (T) nucleotide is substituted for an adenine (A) nucleotide at position 6 of the beta-globin gene's coding sequence. This results in a change from the normal codon GAG (encoding the amino acid glutamic acid) to GUG, which codes for the amino acid valine.