Determination and Maintenance of Cell Type 
ever, in or near an aggregate of around 1,000 or 
more cells, the DSF concentration will be above 
the threshold value. We have purified DSF and 
have found it modified by attachment of sugars. 
We are investigating whether the sugars are im- 
portant for its function. 
We have determined the amino acid sequence 
offour small fragments of DSF and have used this 
information to obtain part of the DSF gene. This 
has been used in turn to obtain the sequence of 
amino acids for part of the DSF protein. Com- 
puter comparison with data banks of other pro- 
tein sequences indicates that DSF is not related to 
any known protein. We are now starting experi- 
ments to see if mammals have genes similar to the 
Dictyostelium DSF gene. 
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