the genetic code is a triplet and is read in a contiguous fashion. 999 bases code for 333 amino acids (999/3 = 333). A deletion at position 901 causes a frameshift mutation from that point onwards. The first 900 bases (coding for 300 amino acids) remain unaffected. The remaining codons (333 total - 300 unaffected = 33) corresponding to the sequence from base 901 onwards will be altered.