def get_neighbors(genotype, mutations):
    """Return all genotypes

    Parameters
    ----------
    genotype : str
        reference genotype.
    mutations : dict
        mutations dictionary from a genotype-phenotype map.

    Returns
    -------
    neighbors : tuple
        tuple of neighbors.
    """
    neighbors = tuple()
    for i, char in enumerate(genotype):
        # Copy reference genotype
        genotype2 = list(genotype)[:]

        # Find possible mutations at site i.
        if mutations[i] is not None:
            options = mutations[i][:]
            options.remove(char)

            # Construct neighbor genotypes.
            for j in options:
                genotype2[i] = j
                genotype2_ = "".join(genotype2)
                neighbors += (genotype2_,)
    return neighbors