Complex sentence: Homologous proteins within or among species are likely, but not guaranteed to have a similar function, and thus comparison of an un-annotated sequence to known homologous sequences is a good starting point for predicting function.
Split into simpler sentences: Homologous proteins within or among species are similar in sequence and are likely, but not guaranteed to have a similar function. Comparison of an un-annotated sequence to known homologous sequences is therefore a good starting point for predicting function.