def most_frequent_kmer(seq, k):
    """
    Function to return the k-mer that is most frequently occuring in the sequence
    Args:
        seq: the genetic sequence
        k : length of k-mer
    Return:
        k-mers that occur the most times in the sequence
    """
    kmer_count = {}
    for i in range(len(seq) - k + 1):
        kmer = seq[i:i + k]
        if kmer in kmer_count.keys():
            kmer_count[kmer] += 1
        else:
            kmer_count[kmer] = 1
    max_value = max(kmer_count.items(), key=lambda x: x[1])[1]
    list_of_kmers = []
    for key, value in kmer_count.items():
        if value == max_value:
            list_of_kmers.append(key)

    return list_of_kmers