The PDx1 protein is found in a wide range of animal species where it is regarded as essential for normal metabolism. In sand rats, the gene coding for the PDx1 protein has a much lower proportion of A and T nucleotides than PDx1 genes from other animals. Finding the complete DNA sequence of the PDx1 gene in sand rats has been difficult. Sequencing involves splitting off nucleotides one at a time from single-stranded DNA. What could account for the difficulty in finding the DNA sequence of the PDx1 gene in sand rats? 1 The PDx1 gene is present in only a small proportion of nuclei. 2 The PDx1 gene is transcribed in only some cells. 3 The strength of the hydrogen bonding between the two strands of the PDx1 gene is unusually high.